Another factor which is very hard to quantify but I feel is very relevent is....
I've noticed that often, when a team picks a successful 1st day pick, they can also 'get on a roll' & have success year after year with said picks. If there is some merit to this, this would skew the results even further due to the compound nature involved.
Examples:
Ravens 96-03
Ogden, R.Lewis, Boulware, Sharper, McAlister, J. Lewis, Heap, Reed, Suggs
Colts 96-03
Harrison, Tarik Glenn, Manning, E. James, Peterson, M.Washington, R. Wayne, Freeney, Tripplett, Doss
Some of the players might get injured or move on but you get the idea.