My unit uses the M2 also and we sometimes have the same issue.
The change in POI ( point of impact )is usually within 2.5 inches out to 50 meters. After 50 meters, I do not see the POI change. I know that they advertise that they are parallax free but I notice that if you use the same cheek weld for a grouping, the group is tight. If the shooter uses a different cheek weld, the POI changes sometimes. Tight group, but different POI. We noticed this on the EoTechs also.
The value of the optics for our use outweighed the " relatively" small variance at these distances.
The Aimpoint advertising says they are parallax free, but the US military manual FM3-22.9
( quoted below ) says not quite so within 50 meters.
CHAPTER 8
ADVANCED OPTICS, LASERS, AND IRON SIGHTS
(Phase V of Basic Rifle Marksmanship)
NOTE:
The M68 is a parallax free sight beyond 50-meters.