That Smell In The Fridge

Why is it that in student houses there is always 'that smell in the fridge'? As students go I'd say we were pretty clean. Messy, yes, dirty no. Our fridge gets cleaned normally once a week, so how is it so that in that time it's able to breed this odour that, at the right concentration, could quite easily be weaponised? Who knows?
I guess the only thing for it is to scrub it out and try again next week.