That is amazing! And after some thought experiments, the only two answers I can reasonably come up with about the settlers limit is this:
1. It's a bug.
2. The game does not actually cap the settlers and charisma. What I mean is this... you can get 20 charisma... but can raise it artificially, which is how some people get 25+ settlers. What if the game doesn't retain that information. So it says Ok, he has 25 charisma now, and an active recruitment beacon, we can send 5 more settlers. But then you change clothes, or the drugs wear off... so you drop back down the 20. The game stops recruiting settlers. But next time you put on a suit and drink some wine, the game re-calculates your charisma... sees you have 25 and send 5 more settlers across your active settlements.
In other words, the algorithm doesn't hard cap the # of settlers at whatever maximum achievable charisma is.
I was just turning it over in my head, like the rest of you - and OP, and that is the best I could come up with.