Have you ever played halo before?
When you play on normal or easy you take less damage, the enemies shoot with less accuracy, and die more quickly. They are slower to react to your presence, and are less aggressive and strategic. They throw fewer grenades. Your shield recharges quickly, and you survive longer without shields.
A veteran player can likely run through normal wilst hardly ever taking cover or stopping to recharge their shields. With 3 teammates drawing aggro, he might be able to play without stopping at all!
In this series, the increasing difficulty increases how long it takes to play through, because one must slow down to survive.
Either you just don't know or you are just being disingenuous... It actually does take more to kill a guy on heroic AND when each enemy gets a performance boost, you have to adjust your play...