Yeah, if the higher ranking chickens want the top roost, they'll get the top roost. If they choose to sleep on the ground, then they either prefer it that way, or can't get to the roost for some reason. For example, if they go in very late, after it's already too dark for them to see the roost well, they may decide jumping in the dark is not worth it, and settle for the ground. I have a couple of chickens that like to stay out late. Some days when they go in, it's too dark for them to see well. I see them circling under the taller roost, craning their necks trying to find a spot, but eventually they give up and settle for the bottom roost. Or, if they fall off at night, there's no way they can get back up in the dark, so they stay on the bottom roost. If you only have one roost, then the ground might just be the only place left if the older chickens can't see or get back up to the roost.