Well, ehm..... On closer inspection, it seems there are actually no more holes on the case that match up to the motherboard's holes...
the far right holes on the mobo are closer to the middle than the holes on the case, so there really seems to be no choice than to use the 6 screws, as you can see in my beautiful illustration:
(x=screws, o=screwholes)
Case: Mobo:
x x o x xo
x x o x xo
x x o x xo
I'm sorry for asking without looking closer, but i was sure the holes would match up. Anyway, thanks for your answers...