The thing is, Kiper and the draft advisory board come at it from opposite perspectives. (I don't really follow McShay's work so I can't comment on it.) "How do you rate this player" and "where do you expect him to go" are different questions that frequently have different answers. Kiper mainly sticks to the first question; the draft advisory board is honor-bound to give a straight answer to the second.
A guy like Locker, who has enormous ability and lacks polish, is the type that generates the most disagreement, particularly with some other name QBs on the board. And QB is the position in the draft that is most sensitive to a team's situation and needs. It's not unheard of for a QB to slip a lot if he gets past the first few teams that need one.