I'll expand a bit.
What I found out is the timekeepers use a second clock in case of errors. For example there's time when the player's movement is the cue for the time clock to start (eg a kick-in) so the player might go but then the umpire blows the whistle and calls them back because of the position on the mark or the other footy is on the ground or whatever so the clock has ticked a couple of false seconds.
When that happens they don't add the time back on to the main clock (the TV one) but keep a second clock which they 'add' the missed time back on at the next opportunity, which is a stoppage.
So if there was an instance of that and the siren came before a chance to add it back on, they would let the siren go for those extra seconds. A very rare circumstance but quite possible.
That's an interesting one and I suspect partly to do with the reduction in normal time in quarters from 25 to 20 minutes years ago. They said that the quarters would not reduce in length because they were going to be stricter in their interpretation of calling time on. That seemed to work and, although my memory of this is vague, the point was to measure the time in play more strictly for various reasons such as not allowing time wasting to impact on the game.
I am not thoroughly convinced that the TV clock is truly accurate down to the second. Hence, as long as we have good sirens (a problem until they replaced the bell with a siren way back and replaced some sirens), then the current situation of the quarter ending when the umpire calls it does not need changing. Surely they have the umpires wired up enough now to receive a signal that the quarter should end.
When that goal happened last week I just thought, oh well, looks to me like he kicked as the siren went but the umpire had not ended the quarter. Clearly the goal should be counted as the umpire is the arbiter of the end of the quarter. I think I thought about it for maybe 5 seconds. Didn't worry me then doesn't worry me now.