Because pressure staus is irrelevant to the fans actual intake/output,
you have a ton of fans pushing air into a case, it will try to expel the excess air through any cracks or gaps it can, keeping dust out.
if you have a load of fans pushing air out of the case, it creates a vacuum effect and sucks in through those gaps, and thats when dust builds up,
I run three intakes to one exhaust and haven't 'needed' to dust my Pc since I built it back in october (I only clean my rig due to Ocd :P)
My Rig's a bit modded anyway and probably not a good comparison for most peoples cases
**Edit, after answering your thread, i saw this a couple of lines down, some nice links in there that may help youhttp://www.tomshardware.co.uk/forum/329241-10-positive-negative-pressure