Apple Watch Sunburn lines when not wearing?

Hi, I recently laid out to get some sun and DID NOT wear my apple watch in order to prevent tan lines... However I got burnt on my arm and the area where I normally wear my apple watch did not get burnt at all? It's kind of concerning. I say another man's post from last year where he experienced the same thing (https://discussions.apple.com/thread/251114194), but no helpful replies or answers.

Why is this happening?