I'm sure there's a reason.
Can someone explain why an AN515-10R screw is 10-24? AN509-10R screw, AN3 bolt, AN365-1032 nut and K1000-3 nutplate are all 10-32?
Just curious.
AN515 has reduce capability over the others with higher threads per inch. It should be used in less loaded applications. My guess is they use less threads where you have less load to make it easier/quicker to install/remove.