"Every frame, they set their horizontal velocity to one of -4 and 4 and their vertical velocity to one of -3.5 and 4.5.
Basically, it's one of these except tweaked a bit to get the bullets to drift down."
I'm not sure how to write these into the fluff just yet. I was thinking that what the player sees is an abstraction of what's actually out there, for the player's and the PC's convenience. This doesn't change the gameplay at all, but provides a good explanation for why the bullets are so visible. The crazy bullets appear a lot bigger than they actually are in gameplay, though. The circle that the player sees is just where the bullet is likely to be at that given moment. It's dangerous, but you can plow through a field of crazy bullets and not get hit.
This doesn't seem like something a lot of players would risk, though. Perhaps area denial is a better role for the crazy bullets? It'd tie in with the fluff, as well. UAVs can't be sure exactly where these bullets will be, so they steer clear entirely.