After knocking around the underbelly of the Orioles’ pitching staff on Monday, the Twins are back alone in first place in the AL Central. That position isn’t new for the Twins in 2017, but it is an anomaly for this decade of Minnesota teams: Already this year, they have ended a day of play in first place 19 times; from 2011 through 2016, they did so on just 13 days combined.

Miguel Sanó is crushing at the plate and Ervin Santana has a top-five ERA, but their individual exploits mask the fact the team is hitting and pitching about as poorly as it did last year, when it lost 103 games and finished nine games worse than any other team in the majors. In 2016, the Twins scored 4.46 runs per game; this year, they’re scoring 4.42 per contest. And compared to last season, this year’s Twins are striking out fewer batters and walking more, while also allowing home runs and hard-hit balls at the same rate.

It’s in the third phase of the game, defense, that the 2017 Twins exhibit signs of a remarkable turnaround. Last year, the Twins supplemented a bad pitching staff with a porous defense, which ranked 28th or 29th in MLB by all three major advanced defensive metrics. This year, the staff is just as poor, but it’s supported by a defense that leads the majors with 27 defensive runs saved — no other team exceeds 17 — and ranks second in fielding runs above average and third in ultimate zone rating. The effect on the Twins’ bottom line is stark: Last year they allowed 5.49 runs per game, just six-thousandths of a decimal point out of last place, but this year they’re allowing 4.58 per game, directly in line with the league average.

The Twins’ transformation from MLB’s worst team to a division leader — albeit one that’s unlikely to maintain its current pace — is attributable mainly to that defensive leap. Speaking of defensive leaps, here’s a Byron Buxton catch from earlier this month:

The former top prospect still isn’t hitting, but his grace in the outfield makes prime Torii Hunter look mortal. Buxton belongs to the rarest and most prestigious clan of center fielders, whose members traverse acres of grass with every bound and are so fleet of foot that they make difficult catches look mundane, and near-impossible grabs look achievable. He can just as easily sprint to snag this liner without diving as he can track back with this full-speed leap at the wall; for Buxton, the outfield doubles as both playground and laboratory, where he can experiment with enhanced difficulties while still completing the catch.

The numbers bear out Buxton’s eye-test impressions. Over the winter, MLB Advanced Media introduced its catch probability metric, which measures how likely a fielder is to catch a given ball in play based on its distance and hang time, and Buxton has excelled by almost any measure using this new leaderboard. He’s caught all 10 of his “four-star play” opportunities — which have a 26 to 50 percent chance of being caught — giving him the most such successes of any MLB outfielder in that bucket. He’s also been perfect on three- and two-star plays — 51–75 percent catch probability and 76–90 percent, respectively — making him the only player with at least 15 combined attempts between those categories to not miss a ball.

Buxton is now Minnesota’s everyday center fielder, and it’s no surprise that the team’s most drastic defensive improvement has come in the outfield, where the 2016 Twins played an out-of-position Sanó and Robbie Grossman — the latter of whom recorded a preposterous minus-21 DRS in left in partial playing time — in the corners. In 2017, though, Sanó is back in the infield, Grossman has seen limited action, and the team is relying more on Buxton and Max Kepler, who both rank among the top 10 outfielders in the majors in DRS thus far.

The personnel changes in the outfield boosted the Twins’ ability to catch all manner of balls in the air, from liners to gappers to rangy fly balls. The effect is noticeable on aggregate, too, and not just on highlight reels.

The Twins have allowed the league’s lowest batting average and slugging percentage on non-homer fly balls and line drives, despite allowing an above-average rate of hard-hit balls in that split. Last year, they ranked among the league’s worst teams by these results despite similar batted-ball data.

On all balls in play, last year’s Twins had a defensive efficiency of .681, meaning they converted outs 68 percent of the time, which was the second-worst mark in baseball. This year, they have a defensive efficiency of .735, meaning they’re turning 73.5 percent of balls in play into outs, which is the second-best mark in baseball. A 5.5-percentage-point difference is a massive amount, on the order of 60 extra outs already this season, or an additional one or two hits stolen every game.

Defensive standouts Buxton and Kepler have been part of the solution, but the Twins also have upgraded simply by avoiding overly detrimental performances; improvement from terrible to average can be just as vital as that from average to great. Consider Sanó, who wields a mighty bat but whose outfield experiment last year ended in unequivocal failure. This season, he has returned to his natural position at third base, and while he will never be mistaken for Manny Machado with the glove, he possesses instincts and athleticism such that he rates as an average (by DRS) or better (by UZR) defender there.

Among all the various player-position combinations the Twins have used this season, only eight have been valued as negative, and none has been worth worse than minus-1 DRS. Added up, those numbers give Minnesota minus-8 DRS when only the below-average players are considered, which is the best mark in the majors. Last year’s Twins, by comparison, accrued minus-93 DRS when factoring in only the negatives, and we can discern that avoiding the negative is important because all the teams that fared best by this measure doubled as the season’s most potent contenders: The Cubs, Indians, Dodgers, Blue Jays, Red Sox, and Giants all placed among the top seven teams.

The final component of Minnesota’s defensive turnaround is its new catcher, Jason Castro, whom the team inked to a three-year, $24.5 million contract in the offseason. The Twins’ chief backstop last year was Kurt Suzuki, who rated 92nd out of 104 qualifying catchers by Baseball Prospectus’s framing calculations; his backup was Juan Centeno, who rated 97th. With Houston last season, Castro ranked third in framing runs, behind only strike-zone deities Buster Posey and Yasmani Grandal, and he brandishes a reputation as a sterling defensive catcher. Pitch framing isn’t accounted for in either DRS or UZR (BP’s FRAA does include the stat), so if anything, the Twins’ leap in those year-over-year rankings undersells how greatly the team’s defensive fortunes have changed.

Cautionary notes around this promising development abound. Defensive data needs a while to settle and is subject to fluky swings this early in the season, and even a league-best fielding unit can’t compensate for all the ills still afflicting Minnesota’s roster, as the offense crumbles after Sanó’s spot in the lineup and the rotation features low-floor, low-ceiling veterans behind Santana and the ascendant José Berrios. But the Twins have certainly improved in a substantial, tangible way. That’s all a rebuilding team can hope for — though a first-place standing a week from Memorial Day is a nice bonus too.

Win-loss figures are current through Monday evening; other stats are through Sunday’s games.