1. When picking up ammo from either an ammo box or ammo cache while the magazine of the weapon isn't completely full, the ammo reserve will go over the maximum amount to account for the missing ammo in the magazine. After then triggering the "decisive reload perk" the ammo reserve resets to the "normal" amount of ammo, effectively decreasing the amount of ammo in the players ammo reserve.

2. When the players ammo reserve is full but the magazine does not contain the maximum amount of ammo, the perk will not grant any additional ammo, even though technically the player does not have the maximal amount of ammo.