The Bills extended Fitzpatrick prematurely (amid a 4-1 start) that backfired on them really badly. I think bad teams tend to overpay for marginal talent in the name of re-signing their own picks (See Kelsay for over a decade). I think Nix and Whaley have made a decision to wait till the player really succeeds. It costs them more, but at least it gives a good feel that he is a blue-chip player.
Levitre was a good LG, but was not a blue chip player. Byrd is a blue chip player and deserves to be paid like a super star. The Bills will pay him eventually, unless Byrd doesn't want to be here.