It doesn't make much sense does it? Why do they move if they already have a nice place and move to a smaller place with fewer amenities?
I just made a small, affordable house in China that was right at $16,000 fully furnished, so that I could drop a few in China and create some new sims to put in China. So I dropped three of those houses and created 6 sims. (2 sisters, 2 brothers, and one man/woman couple) When I get back to the game and my sim starts a new adventure, it turns out that my new contact person is the female sim I'd made who was married. But they were already not living in the house that I had JUST moved them into! They had already moved to a different house that was a lot worse than the one they just came from. Then I went to check on the other two families I had moved in and another one, the two sisters had already moved somewhere else as well.