Didn't you know? Our 'barrels' have rifling in them.
Seriously, I don't know, but I'll hazard a guess.
It could be because it's not a round hole, and you have muscles forcing it out at different strengths, and it could cause the pee to make a corkscrew effect off the sides of the walls.
Or, you may have an obstruction somewhere, like extra skin.
I've read of cases where it sprayed everywhere BUT straight ahead.
(according to women, this is the case all the time in the bathroom)
In other words, fuck if I know.
I do know I used to play target shooter as a kid.