First off I am not an engineer.
The rail is not a complete circle of metal around the barrel nut since you have to have the channel at 12 o'clock for the gas tube/gas block to fit through. Clamping the rail at the bottom of the barrel nut seems like it could create flex in the channel at 12 o'clock. Clamping the rail at the top of the barrel nut cinches the rail around the barrel nut with no room for any flex or expansion. Whether this is actually happening or if it even matters if it's happening is for you to decide. As I said I haven't used a BCM rail yet. And the Geissele rails I do have clamp at the bottom. With a good design it probably doesn't matter but when I see how BCM does it, it just makes sense to me.
I made an ugly paint picture to show what I'm thinking.
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