In Bart of Darkness, we have the following dialogue starting with Bart standing on his treehouse:
Nelson: Hey Bart, your epidermis is showing!
Bart: It is? [Bart looks in vain for the problem, only to overbalance and fall from the treehouse, then screams with a girlish scream]
Nelson: [talking to Kearney as Bart plummets] You see, "epidermis" means your hair. [Bart lands with a thud] So technically it's true; that's what makes it so funny.
It's incorrect because
... epidermis is the outer layer of the three layers that make up the skin, the inner layers being the dermis and hypodermis.
(Source: Epidermis, Wikipedia)
What I'm curious about is why Nelson says this, as it seems just as funny if he said "epidermis means your skin" and more accurate.
Question: Do we know why Nelson says "epidermis means your hair"?
I'm wondering if this was intentional (it may have made the quote more well-known), or if it was just a mistake by the writers.