Giving such discretion to Dept managers reinforces their position, responsibility and accountability for team results. With group incentive schemes, rule 2 is ignored. Those who don't like the subjectivity of allocation tend to leave if they can find and have a choice of other jobs. Individualists may not realise their full potential within group anonymity. With flexible individual bonuses, final bonus details are seldom publicised.
People are more sensitive to differentials than absolute amounts.
Some share information about pay and bonuses and news travels - but disappointment is easier to bear if not proclaimed.