From what I understand (this might be in my FAQ, I dunno), setting your Mii to Mingle means they wander off onto anyone on your list initially, but from there, they might migrate to whoever is on that person's list.
So if I had Ian Sane on my list (whom I would have named as "Grumpygaloob") and I have Marwiio set to Mingle, and then he's got you on his list, it's possible that Marwiio will end up on yours at some time, despite you not being on my list.
I think it can work that way, at least.