I think the responders have nailed it here. Typically that O-rating is strictly stats based, and guys who have been injured or have short careers are going to be undervalued. For drafts purposes this year, though, they definitely tweaked the rankings to more accurately reflect the players' values. This was a new development, I think, as in prior years it was possible to get lots of values based on those artificially low early rankings. Some of us had to work a little harder this year.
Anyway, yeah...ratings probably dropped because now they've reverted to their strict stat basis.