So, this is my understanding... When you have 9 and a campsite, you will not get a 10th villager move-in until you do something to make that happen. That is, either invite them in from campsite, go to another town and recruit someone, etc. So, if you never turn the wifi button on, you will just stay at 9 villagers. (Of course, a villager you have amongst your 9 can still move and you can be down to 8, but you wont go back up to 10).
Now, if you do streetpass people, visit other towns, have guests over, you CAN get a surprise 10th villager.
So, what I do when I have 9 is never turn the wifi button on. No streetpassing, no visits to other towns, no invites to my town. That way, I can be sure there is no surprise.
- - - Post Merge - - -
Well....twice now I've had 10 villagers with one being in boxes. Someone visited my town to "pick up" those villagers and a day or so later, I received one of the villagers that recently moved from their town(s). I didn't think that could happen when my town was full, but it did. I was not pleased by that.
So..I think you must avoid any sort of wi fi/street pass situation if you don't want an unpleasant surprise.
Oh my, that is something I did not know, thanks so much for posting! I did not think that could happen.