Here is a comparison of how many runs the Twins' pitchers have allowed in games this year compared to last year:
11 games >=6 runs
18 games <=2 runs
17 games 3-5 runs
19 games >= 6 runs
9 games <=2 runs
18 games 3-5 runs
In short Twins pitchers have given up 6 or runs in a game 8 fewer times while limiting the other team to 2 or less runs 9 more times. I don't think you need to look any further to understand why the Twins record was 23-23 last year and 28-18 this year. In fact, you might ask why the difference isn't even larger.
Its really pretty simple why the Twins are contenders this year. For the first time in many seasons, no one is in the rotation by default. Far from it. Milone is a major league pitcher stuck at AAA only because there is no room for him. And that is even after losing their most accomplished pitcher in Santana. It shows once again that Tom Kelly was right, you are only as good as your next day's pitcher. And that makes the Twins pretty good.