LAST season the English Premier League (EPL) went haywire. Leicester City—a team of journeymen and no-hopers that had barely escaped relegation in 2015—overturned odds of 5,000-to-one to win the competition, in arguably the greatest sporting Cinderella story ever. At the same time Chelsea, who had ambled to the title in May 2015, plummeted to just above the drop zone at Christmas, and limped to tenth by the end of the season. Jose Mourinho, the Londoners’ high-achieving and abrasive coach, lost his job on December 17th after a defeat to Leicester—who were managed by Claudio Ranieri (pictured), the man that Chelsea fired to make way for Mr Mourinho when he first arrived in England in 2004. Even Mr Ranieri, a humble and jolly man, must have enjoyed the revenge.

The strangely symmetrical fates of these two clubs have reversed again this season, as if the last was merely a dream. Chelsea find themselves seven points clear at the top with eight games to go. Leicester sacked Mr Ranieri on February 23rd with just one point between them and the relegation places (the Foxes have since widened the gap to eight, which should keep them safe). These contrasting trajectories are as unusual as any in the history of English football. If Leicester finish below tenth, they will be the first team to sandwich a title between two bottom-half finishes since Manchester City in 1968. If Chelsea can hold on to first place this year, they will be the first-ever club to finish below seventh in between two title-winning seasons.

What caused these historic blips? Four reasons stand out. First: luck in front of goal. Sometimes a team will run hot, scoring from outrageous positions while somehow keeping out shots that seemed destined for their own net. Sometimes they will run cold, with their forwards squandering seemingly unmissable opportunities as their goalkeeper flails haplessly at easy saves. A simple way to measure this is expected goals (xG), a statistic that estimates the probability of a chance being converted given its position, the quality of the assist and the location of defenders (the data for which is provided by Opta.) Last year the Foxes’ goal difference was 15 better than xG would predict, while Chelsea undershot by four (see chart). The fortunes have now reversed. Leicester are running six goals below expectation, and Chelsea 14 above. Still, streaky finishing can’t explain everything. Take it out of the equation, and Chelsea’s expected goal difference has still risen from 10 to 24, while Leicester’s has fallen from 17 to -4. This brings us to a second cause: N’Golo Kanté. Mr Kanté, a hitherto obscure French midfielder whom Leicester signed from SM Caen for £7.7m ($9.5m) in August 2015, quickly emerged as the chief orchestrator of the lightning-quick counterattacks that propelled them to the title. He demonstrated a remarkable knack for hoovering up loose balls in the centre of the pitch and spraying them upfield to the Foxes’ fleet-footed forwards. All told, Leicester scored 26 of their 56 open play goals in “direct attacks”—those in which at least 50% of ball movement went directly towards goal. Mr Kanté was the first or second man to the ball in ten of them, the highest in the league. He made an astonishing 156 interceptions, 10% more than any other player in the competition’s history. Dan Altman, a former journalist at this newspaper (and now an analyst for Swansea City), calculated that counterattacks in which Mr Kanté participated were 70% more likely to succeed than those without him. Unsurprisingly, the ubiquitous Frenchman soon found himself linked with big-money moves—and ended up at Chelsea, who paid the Foxes £30.4m for his signature in July 2016. Leicester’s potency on the counter dissipated. This season they have scored only six of 25 open-play goals from fast-breaking moves. Mr Kanté, meanwhile, has helped start five goal-scoring counterattacks for Chelsea. That is no mean feat for a team that keeps far more possession than the Foxes did. And when Chelsea do lose the ball Mr Kanté has been quickly on the scene. He has made 66 interceptions, the second-highest tally among midfielders. Chelsea have conceded just three times from direct attacks, compared with 13 last time around. The transfer of Mr Kanté’s efforts has evidently weakened his old club and strengthened his new one. But the contributions of his supporting cast have also varied—a third cause for the topsy-turviness. At Leicester he was aided by Riyad Mahrez, an Algerian winger whom the Foxes signed for a paltry £425,000 in 2014. Mr Mahrez had shown few signs of potential stardom before last season. Yet he suddenly began to produce crucial passes or runs to eliminate the last defenders and finish Leicester’s moves. Last season he completed 35 such actions, which led to six goals. Both tallies were the highest in the league, and helped him win the coveted PFA Players’ Player of the Year award.

The previous recipient of that accolade was Eden Hazard of Chelsea, a feted Belgian winger whom the Londoners purchased for £29.8m at the age of 21. Yet after winning Player of the Year in 2015 Mr Hazard’s form collapsed: last season his forward passes and runs led to just one goal. Superstitious onlookers might have thought that the Belgian and the Algerian had swapped bodies—a footballing “Freaky Friday”. If so, normality has resumed. Mr Mahrez’s creative passes and runs have yielded a single goal in 2016-17. Mr Hazard’s? Six.

A fourth factor in the opposing trajectories has been continuity. Last season Leicester had a more stable line-up than any club in the recent history of the EPL: ten men contributed 86% of the total minutes for outfield players (ie those other than the goalkeeper). Such a feat is impossible for clubs that play in European competitions and must rotate through their squads to handle 50 or 60 matches, as opposed to the 38 games in the EPL regular season. The Foxes had no such distractions, and the fewest injuries of any side in the league, allowing them to build cohesion in the first team (and avoid using their meagre reserves). Once again, the tables have turned. This season, Chelsea have been the ones without European participation, while maintaining 86% line-up stability. Leicester have enjoyed an unlikely run to the quarter-finals of the Champions League, in which they will face Atlético Madrid—a fine achievement, but at the cost of a depleted squad.

The paths that these two clubs have followed are eerily similar. The twinned fates of Mr Mourinho and Mr Ranieri, the contrasting form of Mr Mahrez and Mr Hazard, and the alternating stability of the two squads seem somehow related—as if Chelsea and Leicester had exchanged swigs of “Polyjuice Potion” (which allows the imbiber to temporarily adopt somebody else’s appearance). The truth is more prosaic. The only thing that has actually caused harm to one club while benefiting the other is the exchange of Mr Kanté, who is deservedly the heavy favourite to win the Player of the Year award. That reality matters little to most fans, for whom these two famous anomalies will remain forever intertwined.