Actually I checked this earlier and it was blank on mobile maybe didn't give it enough time to load or something.
Nope, I just had a typo that made that part of data not being displayed on the site
I fixed it minutes before posting here.
pqumsieh has confirmed that accuracy penalty, but if someone is interested where it might be coming from... while it could be somewhat logical to use moving accuracy modifiers even for weapons that can't be shot while moving (otherwise, why would such weapons had modifiers for shooting while moving?), it seems like M3 and M5 have an ability called "m3a1_m5_moving_accuracy", which targets "occupants" and multiplies their accuracy by 0,5.
Can be completely wrong on that, but it seems to be it.