Yeah. You are right. Both are constant stimuli. But, think of it this way – Odour is something you don’t have a voluntary control over. Whatever you do, you cannot smell your own odour (This is also known as olfactory fatigue). On the other hand, you can still ‘un-habituate’ the continuous sound of your ceiling fan and listen to it again. Or a better example, you can still taste your favorite ice cream, though you can’t taste it the way you did for the very first time :) .
