Tool heads can stack, and when stacking they get the durability of the bottom on (the one they were put on). So, once you get one perfect tool head, the rest can be perfect too, just by stacking them, even if you screw the smithing up. The most obvious way to fix this would be to make them unstackable. I'd put pictures, but I don't think they could explain it any better than words, along with the fact that I'm too lazy, so if you don't understand what I mean just go stack some tool heads. Edit: Nevermind, it seems this is already known.