Got some serious man hands there. 
A female friend of mine who is very lean has hands like that, and I once said to her "You have...I can't remember the word. Veiny hands, from being so lean." And she said "Are you saying I have old hands?" And I was like "Well...your hands can become more veiny with age, but no - that's not what I meant."
VASCULAR. The word I was looking for was vascular.

I suppose age does play a role there too, but at such a low level of bodyfat, I suspect that having vascular hands like that is inevitable for some people.