There seems to be something wrong with the Drop Chance of "Bows with Unbreaking III and Power III". I've got three of them in as many days of light play. So, maybe I'm just very lucky, but maybe there is something wrong. Not that I mind, but I thought I should let you guys know ...
I've got five of them now, and I don't have a Looting enchant on my weapon. Too me this very much looks like an error, but if nobody minds I'll gladly collect those nice bows ...
They are a very common drop from skeletons, even the regular skeletons drop them. They are still a 'rare' drop, but It doesn't surprise me that you are getting them. You have been a little extra lucky to get 5, but then, you might not get another for weeks. Nothing wrong with being a little lucky, and they make really good starter bows.