Weird, Huh?
Well, not really if you think about it.
It's where the term "farmer's" tan comes from. Someone who gets tanned on their arms [or lower legs] (but not the trunk of their body) because they're out in the sun in a t-shirt.
Barry, due to his "profession" and coming from the climes of Chicago and D.C. more than likely wouldn't have gone shirtless (or even short sleeved shirt) too often. Thus, only from his neck up got the sun exposure resulting in the tan that's noticeable over his very lite skinned body (trunk).