Question about David's skin on his butt, lol

This is actually a serious question despite the topic. So I'm curious about how the first episodes I believe up until the current one. I've noticed his butt was much darker then the rest of his backside, but this weeks episode when he met his first date, & she commented on on his tan I saw it had changed. It is now even tone, & tanned like the rest of him. When I was in school I was educated on skin, & I know how melanin works, how all races, & ethnicities can have discolorations from extra melanin production etc... Though it's generally on the face, & not too large. So with that can anybody explain what that could've been on his butt, or how it totally disappeared so quickly? I would've said the sun, but that would've made it darker from what I know.