According to your theory the lead sled due to the extra weight on the rest causes a different POI right? So, in theory a person shooting from a sitting position with a bipod from a hard rest should get a different POI than the same person shooting from a prone position with a bipod and a hard rest. According to your theory this would have to occur because the shooter in the prone position would have all of their weight directly behind the rifle as opposed to the same shooter from the sitting position only having their shoulder behind the rifle.
I do believe this to be true. I have a friend who shoots prone competition. He has told me his rifle never shoots the same as it does off the bench. He only uses the bench to work up his load, then his rifle never sees the bench again. But then again, this is a guy that shoots 400-600 yards. Not 100 yards. Tom.