[marcosdumay]

Think about it this way:

1 - My productivity is X per month. (Whatever your definition of productivity, it must be, because I can ask for a bigger salary.) 2 - This other guy productivity is X/2 per month. (By the same rationale.) 3 - The other guy gets a raise, and now is paid twice for the same work. 4 - It follows that I'm losing the opportunity of a raise, because I do more than him.

It's not logically rigorous, it may not be true at all, but it's not an irrational reaction either.