For me the Combine is where the CB ratings get all screwed up. Good CBs who run slow get pushed down and super fast guys who do not have cover instincts get pushed up.
Then after the Combine, the Mocks get down (or re-done) and there always appears to ab a good CB prospect within reach of the Patriots, but come draft day this is always a run on CBs and they go before they are slotted to go.
People who do mocks have got understand that NFL teams are desperate for CB help. It is the hardest position to fill on a team. The college ranks are full of full-length-fur-coat-wearing, prima-donna WRs (who the draftnics love, even on this site), but precious few CBs who can cover.