"seats available" isn't the same as "seats available to be sold".
There are a bunch of people with confirmed tickets who don't have seat assignments (basic economy pax, plus anyone else who didn't get a seat assignment for whatever reason)
This. "seats available" means seats unassigned, not unbooked. It's not uncommon to have 30 or 40 people with unassigned seats even on an MD88 24 hours prior to departure (the highest I have seen is 79 on an MD90). So in reality, you could see that "seats available" number drop from 80 to 0 in 24 hours without a single seat actually being sold.