Those ratings reflect their opinions of how likely/soon players are to start and what their pro bowl potential is. Draft position highly skews position/role, whereas the Scouts Inc numbers do not. So while a guard may grade out as top 5, he will not be drafted in the top 5. Likewise, QB's will be selected higher than their Scouts Inc. Rating numbers reflect.
Pass rushers get selected earlier, but the depth at the top for that role may cause a few with first round grades to slip into the top of the 2nd round... Where we have 2 selections. :-) Also, a lot will change as they play the season and get final combine numbers.