Stuart,
I wouldn't worry about it. It'll hew just fine, you just have to get used to the axe. If it was perfectly flat, it would have a tendency to dig. It's like having a chisel or slick that has a curved back. Only the modern chisels, slicks, and axes have perfectly flat backs for the whole length. It works either way, it just takes a few minutes to get the feel of each tool. And as long as there are two planes cleanly meeting, the edge will be sharp. It looks like a nice axe. If you don't like the curve, let me know, maybe I'll take it off your hands.
