Why does concentrated sulfuric acid burn my skin that hasn't been exposed to sunlight but not that which has?

I got a drop of concentrated sulfuric acid on my arm by accident at one point and it didn't bother me at all. I didn't even notice it for minutes. This surprised me, considering what it will do to plain sugar. Wondering if I had some sort of superhuman invulnerability to sulfuric acid, I tried a drop on my belly and YOUCH! I have a feeling it has something to do with the skin that is regularly exposed to sunlight being different - which is why I never get sunburned on my forearms but if I wear a tanktop for 20 minutes, I'll be red as a tomato starting from where my short-sleeved shirt covers and up. Though it might have something to do with the sulfuric acid drop I tested on my belly I think I got contaminated with phenol. But supposing not, what's the chemistry behind this? Why doesn't it burn my arm?