It's not lag. On a devmap, do g_knockback 0, and get god mode for a dretch and a human. Then, move the human next to the turret, and adjust a dretch so that the bullets just scrub the edge of the human. You will then see some blood.
Turrets do have some spread defined in the source, but friendly fire happens so rarely, that it is to think that turrets should not miss by design. Maybe spread is just for just increased graphics realism only. So what do you devs think about classifying this case as a bug (I do have a fix)?
And if it's any kind of bug that either just annoys anyone, or does crashes, just because it happens on rare occasions, doesn't mean that it should be left in on purpose, especially when there is a fix for it. The only reason one may encourage not fixing a specific bug, is that fixing it would require an exessive amount of time and work, which is not worth the effort. If someone is willing to donate that much effort anyway, one should not try to stop him...