No shows most likely. With no change fees and the like there has been a significant increase in no shows as you automatically get a credit back. That said, I'm surprised that this would be the case on STT-ATL as those trips are typically booked round trip originating from the US meaning that the return leg would also be cancelled upon no showing.

Could also be that the gate agent forced (or negotiated with) families / couples to sit together which opens up more seats given the seat blocking. One family of four will take up the entire left side of a 738 but could consolidate into two rows, leaving up to 4 seats free if the next four folks on the upgrade list were couples.

Was the flight full / oversold?