Why did't they just use the one the air force has. I never understand why they do things like this in the same country. Just share, its better in every way.
The reason for this (besides politics, big ego's, separate budgets) is that you simply cannot compare the field conditions of a flak/AA crew with that of a navy vessel or air force base.
AA/Flak crews might need a helmet that is lighter (so you can run around with it). Or perhaps they need greater comfort for groundfighting/close quarter. Either way, field conditions are different, hence design philosophy will be different.