Perhaps the 7MM RM C-L is constructed a little stronger for larger game at longer ranges? I've never taken one apart??? They sure didn't expand on lighter Whitetails at <100 yards!
I'm still at a loss to explain it!
The difference is in bullet construction, the jackets are "drawn" out differently to have specific tapers and thickness'....That plus other things makes a difference in how fast ect.. they expand...