Becuase unlike other games where ammunition is magically stored as a total number of bullets, RO models the fact that you are carry a certain number of magazines.
So if you reload your MG before emptying the first drum, that drum with a few rounds in it goes back in to your inventory. So eventually when you reload again, it will be put back into the gun, albeit with only a few rounds left.
This is unlike other games which simply model how many rounds your player has with them, not how they are stored (ie, telling you you have 30/120 rounds)