This behavior is moderately annoying for those who are at the top of the upgrade list, i.e., 1Ks and maybe GS. When they run the list en masse down to J2 or whatever, it leaves little possibility for swapping seats. For example, if the computer decides to assign seats front to back, I might clear at #2 into 6D or whatever on the 77W, whereas a Silver might clear at #20 into 15A, which is a far more desirable seat, at the same time. If they actually bothered to clear folks on these wide open flights at their respective windows or at least in phases (GS first, then 1K, etc.), it gives those at the top of the list a shot at getting the seat(s) of their choice.