I had a rummage through my stash and bolts time and discovered something about the avid ti, mag and sd7 brakes I had not noticed before. The bolts have a gradient that fits into a chamfer on the barrel. It's slight but I get why. On some brakes the barrel is slightly proud of the arm, so a flat bolt can be used, but on the avids it's all flush so a flat bolt would potentially bind on the brake arm.
Interesting. Every day is a school day.