Do you have an example where fares start high and come down? Are you outside the Northeast Corridor?
NYP/BOS tomorrow (Jan. 23) is between $20 (night owl) and $83, with the regular low base fare at $32 still available for some trains.

For Dec. 20, 2024, it is much the same, although there are more $63 fares and fewer $32 or $83 fares.

To me, that supports my understanding that it's solely about availability.

Of course, Fridays are different. The high fare this Friday is $139 (172) and I would not expect that to drop.