Sorry guys, but I have to laugh since this happens all the time in my game and I swear that the game deliberately causes the sims I want to be a certain way to either pig out at the vendor stalls or live in the gym. Really frustrating at times but it's nice to see I'm not the only one.
What I have noticed and thought I would share is that in my save, the sims that keep showing up at the gym seem to be the ones in households that I played recently. Not necessarily the ones in the previous household, but those a couple of household before. Who comes and goes does seem to change over time. What gets me is the sims I want to see at the gym to keep their fitness level never seem to show up there. It's always seems to be the ones I don't want to work out. From what I have seen, there is no method to the madness. Traits and fitness levels don't seem to have much impact and I've pretty much written it off as just random. But the gym isn't any different than any other venue as far as I can see. It's always the married sims that show up at my bars, etc. and very few if any of my single sims when I'm trying to find a partner for a single as an example.