Dividing teams into tiers is one way of going about this; it's also the wrong way. We're talking about a system that lumps the Red Sox and Twins into one category! I'm rooting hard for the Rays down the stretch; if their schedule included 40 (out of 55) games against the "playoff-bound" Twins, I'd be ecstatic about Tampa's chances.
In my opinion, the Twins do not have the easiest remaining schedule in the AL Central. The Tigers do, because instead of having to face Detroit--as the Twins do--they play three games against Minnesota. That couldn't possibly suggest any flaw in the methodology, could it?
If you're curious, here are my estimates for each team's strength of schedule from here on out. The numbers have been adjusted for home/away games.