It's been weird for a while, even in the vanilla game. It is possible to get migrants even if your population exceeds the cap, but based on the Mantis discussion in relation to this "bug" it looks like there's a difference in how the setting is expected to work and how it actually works. The expectation is that it is purely a performance feature independent of gameplay, but in reality it seems that it still has to go through the "communication" that's simulated between your fort and the mountainhome via the liaison. That means that even if you're down one below the cap when he last left, you could get as many as three or four waves. On top of this, migrant waves aren't really controlled except by fortress wealth (and possibly others like danger/deaths?) - so if you're short of the cap, you're getting migrant waves, and those waves can be anything from 1 to 20+.
Someone had suggested setting the cap to 0 and it seemed to work, but take it with a grain of salt since there was no mention of rigorous testing. You'll start with your initial 7 and, baring any other bugs, maybe won't get any more.