Well it could be cant but that must be huge to make quite that much difference and pretty obvious. Use a spirit level?
Trigger action? If there is a slight upwards motion on release then that would throw it higher. You could test that by taking turns holding and pointing but with the other guy releasing the shot via a cotton thread tied round the trigger?
Only other thing that comes to mind is that your friend is winding you up something rotten and aiming off :-) Surely not!