Think of the 800 units as a pool from which Alive can draw.
The number that spawn at any one time is given by the spawn limiter. If we set that to 1 (=1 group), and only had 1 objective, we would get between 4 and 8 units spawning at that location.
If you killed them (let's say 8 ai dead), there would be 792 left in the pool, but only another 4-8 units would then spawn again in the same place.
Of course, in the mission there is more than one mil objective, even with the filter applied, and we're spawn limiting at higher group sizes. Taking the filter off, or reducing it, can reduce the probability of ai spawning at that one location, if all ao's are active, but it also causes the Alive ai commander to have too many objectives, which kills performance.
Perhaps that's already clear to you, in which case ignore.

But perhaps not everyone will have understood the difference between the pool size and the spawn limit.