User blog comment:Blood Reaper/Concept Art from Skyrim/@comment-67.85.82.244-20111004042421/@comment-153.26.178.60-20111004124915

Both. Anytime you use a particular skill, you get better at it. So by casting the spell to conjure it (or scroll) that would increase as well as archery when you use the bow. like in previous games, bound bow will likely fortify your archery skill temporarily as well.