Simple...guys like Kiper & Mayock rank all the players before the draft and then hand out post-draft grades based on how teams drafted versus where they had them in their rankings. Nothing more, nothing less.
I'm not sure why guys like Kiper and Mayock get ripped so much. The other talking heads that participate in the draft coverage who should be shown the door are guys like Keyshawn, Mort, Jamie Dukes, etc...
Do you really believe that these folks put in the same number of hours that a Kiper or Mayock do? The draft when it was Kiper and Berman with the occasional cut to the studio or for an interview was much better than the 5 person sets that both the NFL Network and ESPN are now using. The draft shows now follow the philosophy of the pregame shows...throw 4 or 5 people on a set and have them all talk over each other and yuck it up.