Since their incredible World Series run in 2016, the Chicago Cubs haven’t always lived up to the grand expectations set forth for their young core. The 2017 version never really clicked, even before the Los Angeles Dodgers dispatched them in the National League Championship Series, and this year’s club spent almost all of the first half trailing the upstart Milwaukee Brewers in the NL Central race. And yet, the Cubs’ empire hasn’t crumbled yet. They recently fought their way back into first place, having won 15 of their past 20 games through Sunday’s win over visiting St. Louis, and our projection model has them tipped as the NL favorites.

Unlike the 2016 Cubs, who enjoyed a historically effective pitching staff, Chicago’s primary weapon this season has been a dominant offense that is averaging an NL-best 5.2 runs per game. At a glance, that might not be too surprising for a team with Kris Bryant and Anthony Rizzo penciled in the lineup every day. But the driving forces behind this year’s offensive performance aren’t the leading names we’ve come to expect — and it remains to be seen whether this new configuration is sustainable as the team moves away from the All-Star break.

From 2015 through 2017, there essentially wasn’t a better 1-2 offensive punch in baseball than Bryant and Rizzo. Together, the pair was responsible for 712 weighted runs created (according to FanGraphs.com) — the most by any two teammates over that span — and they generated 30.5 percent of the Cubs’ total runs — which, among duos, ranked second to the Los Angeles Angels’ Mike Trout-Kole Calhoun tandem. To the extent that Chicago was a legitimately great offensive team during those seasons, that was mainly the doing of the Cubs’ top two bats.

The Cubs are getting help from more places Top five Chicago Cubs players by share of weighted runs created (wRC), through July 21, 2018 2015-17 seasons 2018 Player Share of Cubs’ wRC Player Share of Cubs’ wRC A. Rizzo 15.3% – J. Baez 11.8% – K. Bryant 15.2 – K. Bryant 11.0 – D. Fowler 7.5 – W. Contreras 10.4 – A. Russell 7.1 – K. Schwarber 10.2 – B. Zobrist 6.4 – A. Rizzo 10.2 – Source: FanGraphs

This season, however, Chicago’s hitters have spread the contributions around. Through Saturday’s games, Bryant and Rizzo had generated only 21.2 percent of the Cubs’ weighted runs created, with an even greater share turned in by other tandems on the roster — such as Javier Baez and Willson Contreras (22.2 percent).

For Rizzo, it has been an outright disappointing year. Going into the season, FanGraphs projected a weighted on-base average (wOBA) of .389, but through Saturday he had only managed to deliver a .337 mark, which is barely better than league average and represents his worst output since 2013. Rizzo’s power numbers are mostly to blame; he’s on pace for just 20 home runs after averaging 32 over the previous four seasons. Although StatCast indicators such as exit velocity suggest that Rizzo has been hitting the ball plenty hard, his swing has flattened out into more of a line-drive stroke, which has resulted in many fewer balls hit in the sweet spot for power and launch angle.

Bryant has been much better than Rizzo, with a solid .371 wOBA that ranked 26th in baseball. But even that comes off as slightly unsatisfying after a 2017 season in which Bryant was a top-10 hitter and set career highs in batting average and on-base percentage. At 26, Bryant was supposed to be on an upward trajectory toward peak production, but this season has been a step backward — particularly given that his rates of hard-hit balls also have been trending in the wrong direction.

Luckily for the Cubs, others have stepped in to pick up the slack. Baez is enjoying easily the best campaign of his career, and he has the potential to flirt with 30 homers before season’s end. Contreras, too, is tracking for his finest season (thanks especially to an uptick in playing time), and Albert Almora Jr. and Kyle Schwarber had a combined .351 wOBA after going for a .333 mark last season. Even Jason Heyward, whose struggles at the plate have held him back when compared with his star reputation in the field, boasted an above-average .337 wOBA. Not every Cubs player is having an outstanding year with the bat — sorry, Addison Russell — but most of them are: Of Chicago’s top 10 batters by plate appearances, nine (all but Russell) are above the league average in wOBA.

Naturally, the Cubs would love for their lesser stars to keep raking while Bryant and Rizzo revert to their previous form, although underperformers don’t appear any more likely to progress upward to their previous means than overperformers are to slide back downward. At least, that’s according to a quick and dirty study I did of qualified hitters since 1950. The 20 percent of players who overachieved most (in terms of wOBA, compared with that of their career previously) tended to give about 61 percent of their gains back the following season. Meanwhile, the 20 percent who underachieved most tended to regain about 67 percent of their previous form the next year. The difference wasn’t quite statistically significant — and that’s before considering that the sample is somewhat biased for underachieving players to bounce back. (Remember, they needed 250 plate appearances the next season to be considered.) So the Cubs should be prepared to take the good with the bad when it comes to which players revert to their previous standards of performance.

Either way, Chicago will need to keep scoring runs. As Craig Edwards of FanGraphs wrote a few days ago, the Cubs’ starting pitching has been so bad that it’s on pace to accrue the fewest wins above replacement in recent franchise history. Chicago’s bullpen has been reliable enough to salvage some victories, and its defense is very good, per usual. But barring some kind of major trade to shore up the rotation, the Cubs will continue to ride their offense-first philosophy into the postseason and hope whatever regression comes is of the positive variety — such as Rizzo and Bryant turning into the team’s dominant combination of old — rather than the negative.

Check out our latest MLB predictions.