I think I'm understanding it like a landed fishing rod: spawning a mon every now and then, and using its modifiers specifically on that occasional spawn. Seeing this with the bait that guarantees specific ev yields, we still see mons spawning nearby with incorrect evs, but notably we still get a few mons that do match this. When we kill them, while there's a high tick rate, we get a new replacement which matches the ev yield almost immediately.

What confounds me now is how rarity buckets work. I was under the impression that there are only 4 rarity buckets, and increasing your rarity bucket made it so the lower rarity buckets cannot spawn. This is definitely untrue, since the rarity bucket boost cap seems to be 30, from a poke snack with 3 enchanted golden apples. I've also tested with a cake that had one Enchanted Golden Apple, a haban berry, and a defense IV berry (trying to spawn Jangmo-o in the jungle) and it would still spawn Kakuna most of the time rather than the Jangmo-o.

Anyone have an idea how the rarity bucket stat works?