If you suspect they're floating, detach the line coming from the compute terrain to the population, and have it go from the last node before it goes into planet to the population. That way it picks up all final displacements (with the danger they also sit on fake stones, so if you have fake stones in the last node, take the one before that to have the pops sit upon).
Details look ok, a bit grainy clouds maybe, and maybe due to the atmosphere quality, you might need to increase that from 16 to 24 or so.
And do you have a slope restriction on the snow? It kind of clings to very vertical areas, so you might want to refine that...