I started a legacy recently, the pizza girl and the postie looked very similar to the one you pictured above, and I had a couple of attractive, well dressed teens go past my front door too, from "not in world families". Needless to say, my bachelor Sim quickly befriended them to stop culling and was most relieved when their families moved into his neighborhood (her Mom wasn't bad either!).
When she aged up (ok, this is seriously creepy behaviour, but it's just a game remember) he immediately moves her in and gets to work on the romance. Teen #2 is kept on as an "arranged marriage" for his heir.
With all of this scandalous partner hunting done, i stopped paying attention a while until i saw a pair of young adults taking selfies outside my lot. Normal service has been resumed, the Dude had black skin, left parting comb over with sideburns, a suit jacket, hot pants and was wearing cowboy boots. His love interest was a white woman with a ginger mohican , enormous spectacles, a vest top, underwear only below, and pink heels. Both were extremely fat.
I can only conclude that in successive patches, the Sim population has been seeded with families Maxis have liked from the Gallery, but if you play long enough these run out and you're back to the good old Random Number Generator.
I need two more points to become a full member so i can actually show you pictures of this horror show.