Not intended to be an insult, but I do not believe that it is your friends rifle that is shooting to a different POI from the different positions. I believe it is him. Some shooting positions are naturally more steady than others. Once you move towards the more unsteady positions human error is again involved.
Personally, I have not noticed any difference in my shooting out beyond 400 yards from the different positions with or without a rest that cannot be attributed to human error.