It's probably a pre-integrated DM from two possibilities, mix 'n' match:
1. Missiles are significantly smaller targets than spaceships, which is only partially offset by being at point-blank range.
2. The turret is probably slewing around much faster than is usual in regular direct fire (which typically involves adjustments measured in fractions of milliseconds of arc), again due to the missile's close range, but also due to the possibility of one turret engaging up to three missiles (depending on number of lasers installed).
Unresolved is whether pulse lasers would add their typical -1 To Hit DM and whether fixed mounts would add their typical -2 To Hit DM... as well as what an appropriate To Hit DM for shooting at missiles not "in contact" would be for regular direct fire (although -3 seems a popular assumption that I've often seen).
Also, there is an obvious opportunity to write an add-on piece of software for Anti-Missile that *would* allow Gunner skill and Predict DMs and so on... and while your PCs are coding, might as well extend it to work with the ECM program as well... gives the excuse to add a dedicated EW officer with Gunnery and Electronics/Computer skill to the payroll.