What would cause only one of my cat's anal glands to not be expressed?

Got my boy's anal glands expressed today and the vet said one was empty, but the other was waaay full and probably causing his discomfort. It definitely was, when I got him back he was purring and relieved. Just curious, why would one express itself via normal bathroom habits and the other not? Is there some way I can help him make sure this doesn't happen again?