The less features a board has the more room there is to fit in CPU compatibility. The A*20 boards don't have to support the board chipsets that a B*50 or X*70 has because it relies only on the SOC's functionality in that regard. A series boards also typically don't support many features that the other lines do including just basic CPU overclocking. The bare bones approach actually allows for a greater compatibility range, if the motherboard maker is willing to put in the resources to make sure everything works.
OK this MB has a 16MB bios, how it this possible? if i say what im thinking right now half of the forum is going to insult me, so i want to know how this is even possible? O already knew something was off because the Asus A320M-K never dropped Bristol Ridge support and it supports from a A6-9500 to a R9-3950X on a 16MB bios.
Edit: After re-reading your post I see you are talking about the B450 board and just mentioning the A320 board as an aside. For the B450 board you mentioned, they basically split the BIOS so if you are using a pre-Zen 2 CPU you are supposed to use the older BIOS revisions. If you are using Zen 2 or Zen 3, you can use the newer ones. That's the compromise that's being made to support everything.