The issue with a ranking system like this is that it really doesn't mean much for the purpose of evaluating team talents... I think it's just there to have something like "power ranking" that's kinda popular in the media, to just feed people with something to talk about.... (...see i'm really jaded.....hahaha)
I think the team ratings are derived from player ratings. When you play franchise and look at team ratings before and after spring training, you see the rankings can be drastically different. I'm assuming that's because pre-season, the team ratings are derived from all players (don't know if that's just 40 man or all including minors), but after the spring training and 25/40 man rosters are done, then the team ratings probably are derived from the players on those more limited roster.
So team ratings aren't really what many think they are...