pre draft rankings are only a limited indicator in regards to actual draft position. There may be a pair of DT's and an OT that are the best three players in the draft...1,2,3... but if the first three teams need QB's,...and there are three QB's in the top 15 or so...then they are going 1,2,3...and the three best players in the draft drop.
also...maybe there are only 4 decent OLB's in a draft...with 2 ranked as first rounders and 2 ranked as second rounders......if by pick 28 the top three are gone, said team at 28 may go ahead and grab the last one to make sure they have one ...rather than chance he will be gone by the time they pick in the second.
I like walter football and CBS...draftek is good as well....but with any of them it is better to break down into positions then rank the top players yourself.