This appears to be true but must be unintended if this is the cause. I think it's an outlying bug with the accidental friendly fire code (which went through several iterations, mostly for the better, years ago).
It really should be either you do enough damage to a non target or damage a target and it's then either you get a bounty or you don't. It seems the sub/function that returns agro isn't in synch with the one that applies the bounty.
I'd guess it's a rounding error or a legacy constant
They can also accidentally hit you, which, on a turret boat can make your ship consider them to be a threat and return fire.