Oh nice one, I've wondered that a few times myself.
At Schiphol, as far as I can tell, there's some kind of rough division of the gates into groups designated by a letter which I think refers to one of the...oh what's the word...'leg of the spider' if you will.
Oh! Apparently they're called piers. I should have been able to guess that myself:
The budget airlines have H-gates, and the pax get lots of exercise walking up and down the steps
I
think KLM and BA flights usually go from (and arrive at) D-gates. I once flew Swiss but don't remember the gate no
So maybe the company rents the potential use of a bunch of gates, and then which aircraft goes where at arrival is determined by more immediate factors of which gate in that group is open at the time?