I'm not 100% sure on this, but I think it has something to do with certain factions not actually using slaves.
For example, if you pass an AI controlled mine and click it, you will see slaves inside, if you cap it, you get those slaves. Other times, with different factions, you will see they only ever use workers, which rout when you cap the mine.
So my guess would be either the AI starts with all the slaves it needs, so doesn't need to capture more, or just one parameter was used in which all AI executes rather than different ones for slavers and non slavers.