Yeah, I haven't seen that happen either, so it must have been a coincidence. Also, every time you leave the Able sister's shop and come back, Sable is working on a different design, so it must be random.
I think that applied to the AC older games, but not New Leaf (going off your observations) because I have heard about this since the GameCube days. I could be wrong though so clarity from someone who has indisputable proof would be great!