I had actually logged a bug for this last year. Case number 5817532.
Plarium does have an explanation, and has said it will make things clearer but evidently that hasn't happened yet. Here are excerpts from the explanation:
"Quantum Gun fires 2 bullets at once and can shoot up to 12 times per Magazine.
These mechanics are more clear when using the Weapon. For instance, when you fire Quantum Gun once, the magazine will be reduced by 2. Also, 1 bullet is yellow and the second one, blue."
In short, their explanation is that the damage per round is actually the damage per pair of rounds (one blue and one yellow), so for full magazine damage you multiply it by 12 instead of 24.
The confusion of course is in their inconsistent definition of the word "round". It is a pair of rounds in damage but individual rounds in magazine size.