If you look at runs scored and runs allowed, there isn't a big difference between the Tigers in 2010 and 2011. Last year, the Tigers scored 751 runs and allowed 743 for a run differential of +8. This year, they have scored 639 runs and given up 628 for a +11 run differential. A run differential of 10 is worth one win on average, so you would expect both teams to be one game above .500. In actuality, this year's squad is much better than that:
2010 81-81 .500
2011 77-62 .554
So, two teams with almost the same run differential have a .054 difference in winning percentage. How does that happen? Often times such a discrepancy can be explained by a team's performance in close games, so let's look at that. The Tigers have been much better in one run games this year than last year:
2010 16-26 .381
2011 24-15 .615
That's a 9 1/2 game difference between 2010 and 2011 which is substantial. Here's what would would have happened if they played .500 ball in one-run games both years:
2010 86-76 .531
2011 73-66 .525
The winning percentages would have been almost identical, actually slightly better in 2010. So, the difference between 2010 and 2011 can be explained by performance in one-run games. Why does a team perform well in one-run games?
In 1999, Baseball Prospectus co-founder Rany Jazayerli and former Baseball Prospectus writer and current researcher for an MLB team Keith Woolner studied the relationship between bullpens and records in one-run games. They discovered that teams with good bullpens tended to exceed their expected record in one-run games while teams with poor bullpens were more likely to under perform in one-run games. Their study does not explain all the discrepancies between actual and expected win totals, but bullpen quality is the biggest factor that has been found to date.
Now, the Tigers bullpen has had problems getting through the middle innings this year, but they have had no problem closing out games. Jose Valverde is 40 for 40 in save opportunities which has probably boosted their record in one-run games.
So, is Valverde's perfection in save situations the reason they are better this year? That's probably part of it. It could also be better clutch hitting, better managing, better luck or any number of other reasons. Whatever the reason, it goes to show that there is not necessarily much difference between a .500 team and a team on pace to win 90 games. Sometimes, it's just a run here and there.