Putting a mod onto a barrel will typically shift the poi down due to weight and changes in the harmonics of the firing cycle.

HOWEVER... it can also send the POI up or left or right or ANY combination of these.

You rezero to allow for this shift and remember the POI will again shift if you take it off.

I also generally find the group may tighten a little but as the bore of the mod and the barrel may not coincide...there is no guarantee it wont open up either. The other possibility is clipping but this is usually pretty obvious due to not being able to hit a barn door and shaving in the mod etc.