I actually do think it’s a random occurrence.
For the ai as well as the human player.

I experienced this a number of times.
No matter who’s got the higher troop size.
I remember ones attacking Chartage I had to fight 2 full stacks with my semi-stack.
But the probability reinforcements delayed depending on your troop superiority is indeed influenced by it.

Ofcours I could be completely wrong about this.