I suppose it's a case of Early Installment Weirdness and Characterization Marches On.
Another example of odd, early Brain was the episode, which title escapes me because I rarely ever saw it on TV, where Brain was sleeping over at Arthur's house, and he was all neat, polite, and well-behaved, but we saw that at his own house, he's actually quite a slob. And, again, like the comic books and trading cards (I remember he collected baseball cards in "Arthur's First Sleepover"), it's never mentioned or showed ever again. Then again, characters like Brain are usually depicted as being compulsive neat freaks who are OCD about cleanliness and organization - Double D from EEE is another example. Dexter is another subversion, however, there was that one episode where he hated washing his hands and taking baths, so he had himself laminated so dirt and grime would just slide off of him.