Very simple, Garrison Joe. The Wild Bunch Rules Committee has said so. If a stage calls for 4 shotgun rounds, you load 6, jack a live one out and shoot the next round loaded, it’s illegally acquired ammo.    
People would only ever want to load eight in a magazine because others keep perniciously,  ignorantly or arrogantly telling them it’s perfectly fine to do so.  
Now, GJ, the question has been asked and answered.