I had been looking into this (but couldn’t post here since it’s locked).
I’m still checking into it further, but I think @Zane_Milakovic is correct about the closest region. So if your stopped Machine is in the closest region to a user, then a request could get routed to it even though it’s stopped and the other Machine is below the soft_limit. This behaviour is where load balancing intersects with the auto start and stop feature. (Hoping to update docs for this soon!)
But if this Machine is restarting on a very regular schedule that doesn’t match with incoming requests, then something else might be going on.