They have different zodiac signs, different personalities, different appearances. In the neighborhood I am currently playing I almost always have multiple births in each game session. I have used the lot debugger randomizer feature. Nothing changes it. ALL sims have a 4 in health. ALL adult sims have an 8 in fashion, work and weather.
After reading this post I've been checking the interests of my Sims as I play, and out of the 13 BIG sims I've played so far, 12 of them have a 4 in health/8 in weather, work, and I *think* environment.
I'm trying to figure what distinguishes the one without those exact interests from the other twelve. All I can come up with is that he's a twin, and he is by far the oldest Sim I've checked so far. I play quite slowly and it is very possible that he was born ages ago with only the base game installed.