I guess I had never thought about the fact that, because the shot clock (unlike the game clock) is not equipped to display tenths of a second, then (if there were no internal adjustment) when it got down to .9 seconds, the shot clock would display "0" for the next .9 seconds, at the conclusion of which interval the buzzer would off. Having the shot clock show 0 time left for a full .9 seconds would be confusing and weird. So what they have done to avoid this is to set the clock up internally so that it really starts at 24.9 seconds, even though it is showing 24 seconds. That way, as soon as the internal clock goes to .9 seconds (i.e., at the very instant the full 24 seconds are used up), the shot clock goes to zero and the buzzer goes off.

The fact that the shot clock does not display tenths of a second also means that when the shot clock is showing, say, 2 seconds left, we don't know whether this means that there is a full 2.0 seconds left to shoot or 1.1 seconds, or something in between. I wonder if this imprecision causes miscalculations by players that leads to shot clock violations.