Year after year, teams go through dramatic changes. This year’s veteran champions become next year’s aging also-rans. Last year’s callow group of rookies becomes this year’s energetic contenders.

It’s tough enough figuring out which teams are most likely to change from one year to the next. It’s next to impossible to figure out how those changes might take place.

Three of this season’s top playoff contenders have seen a big change in a key aspect of their respective ball clubs. A year ago, these were Achilles’s heels; now, they’re bona fide strengths.

Phillies Bullpen

Baseball analysts love to deride teams for spending big bucks on relief pitching. When future stars like Joakim Soria spring up for the cost of a Rule V claim, it’s tough to get behind a multiyear, megabucks deal for a garden-variety setup man.

Fortunately, Phillies General Manager Pat Gillick probably hasn’t listened to a single analyst in his entire life. This offseason, Gillick signed J.C. Romero to a three-year, $12 million contract, rewarding the veteran lefty for 36.1 sparkling innings in Philly last season — a year after putting up a shiny 6.70 ERA. Gillick then made a deal with Houston to acquire highly paid closer Brad Lidge, later signing him to a gigantic three-year, $37.5 million deal.

Doubters came out in full force, especially on Gillick’s move to make Lidge the team’s closer. This was the same guy who’d gone from “Lights Out” Lidge, the slider-hurling beast who plowed through the National League, to shell-shocked Lidge, the pitcher who surrendered a heartbreaking, game-winning homer to Albert Pujols in Game 5 of the 2005 NLCS, an event which led to an awful two-year slide and a crisis of confidence. Gillick, and maybe Lidge’s wife, were probably the only two people who figured a homer-prone pitcher with a history of mound meltdowns would fit in with the generous hitter’s dimensions and often abusive atmosphere of Citizens Bank Park.

Both Lidge and Romero have passed their tests with flying colors, leading the team in ERA and totaling 130 strikeouts in 119.1 innings. Combined with lanky righty Ryan Madson and cheap offseason pickup Chad Durbin, the Phillies have gone from having one of the game’s shakiest pens to the best quartet of relievers anywhere. That vastly improved bullpen is one of the biggest reasons that the Phils are back in the hunt for first place.

Rays Defense

When Stu Sternberg, Matt Silverman, and Andrew Friedman arrived on the scene in Tampa three years ago, they brought the principles learned from their Wall Street careers to a team badly in need of direction. But in their first two years at the helm, it was tough to decipher a pattern in the Rays’ approach. The team seemed to be doing little more than stockpiling as many potentially undervalued assets as possible.

This offseason, that all changed. The showcase move was a six-player trade that sent Delmon Young, Brendan Harris and Jason Pridie to Minnesota for Matt Garza, Jason Bartlett, and Eduardo Morlan. In one fell swoop, the Rays had remade themselves into a squad that would focus heavily on pitching and defense. Garza joined a promising, young rotation that included Scott Kazmir, James Shields, Andy Sonnanstine, and Edwin Jackson. This was a staff with a chance to succeed — if the guys behind them could make some plays.

Fortunately, the Rays have fielded a vastly different group of glove men this year. The big trade subtracted two of the team’s worst defenders, Young and Harris, from the everyday lineup. Rookie third baseman Evan Longoria joined the team soon after Opening Day, bringing his stellar glove to the hot corner while also moving Akinori Iwamura to his preferred position, second base. Just like that, everything changed. The Rays rank first in MLB in Defensive Efficiency — the percentage of balls in play caught by defenders — at 71.8%. That’s a quantum leap from last year’s 66.2%, the worst figure in baseball.

Put another way, Rays opponents are hitting just .282 this year on balls put in play this season compared with a cartoonish .338 last year. That’s the biggest defensive jump that any team has seen in 50 years. It’s also a huge catalyst for the success of Tampa’s young pitching, and the team’s drive from worst to first in the AL East.

Cubs OBP

Last year: .333, 18h in MLB

This year: .356, 2nd in MLB

So what happened? Breakouts by several players at once. Geovany Soto’s got a great case for Rookie of the Year honors, posting a .285 batting average and .502 slugging average, while getting on base at a .365 clip. Newly acquired Kosuke Fukudome hasn’t hit for much power, but he has delivered a solid .360 OBP. Mark DeRosa’s sitting on a .375 OBP, a number that would mark a career high and his fourth-straight season of accelerating on-base results.

The biggest shocker has been shortstop Ryan Theriot. The Cubs probably would’ve settled for some good defense, a bunch of steals, and a few timely singles from the slap-hitting 28-year-old. Instead, Theriot has been a minor star, hitting a robust .306 with a gaudy .385 OBP, tops on the team. Once a team that relied on starting pitching and power hitting (not unlike their South Side rivals), the Cubs are now much more balanced, putting scads of runners on base ahead of the big boppers. Is it any wonder they own the best record in the league?

Mr. Keri (jonahkeri@gmail.com) is a writer for ESPN.com’s Page 2.