They didn't talk about it explicitly, but in my mind it was connected to how the guy who got injured felt progressively better each time he went down a dream level. Dreams are affected by what happens above them, but it's sort of passed through a filter.
Exactly, the dreamers in the van are feeling 0g, so they're dreaming in 0g as well. However, it's still a dream so when you reach the dream-within-a-dream level, the brain probably defaults to what it's more used to.
