F=Ma. Does the rate of loading come into play? When you pick up the grocery bag slowly, no problem. Yank on it and the groceries rip out the bottom. All spokes I have broken have been straight gauge. 1 butted spoke lost in 10 years. Being over 200 pounds I am surprised I have not broken more.

Also, I am sure that some hub drillings might affect general wear at the head.