And I have a hunch it was structured that way to open the possibility of eventual expansion, like a "test case" (a step set initially when it expanded to 65 in 2001(?), although I think that happened because # of leagues increased so the power teams felt it needed to maintain the then-current at-large slot).
It was almost like a compromise that kept the basic 64 framework w/ lowest auto-bids at 16, and "narrowly miss" at-larges in 11-seed area.
If four more at-large wannabes are added, moving it to 72, it'll shove all the lowest auto bids onto 16 line (so we'll have a First 8 split between 16 and the current at-large lines).
I realize that's probably not the most well-constructed analysis, hopefully it makes some sense.