As I pointed out over there...
If you have a scope which is not correctly PA'd to the range your shooting at, even if you have consistent head position, it's possible that you could be shooting consistently with error, ie constantly offsetting the reticule from it's correct alignment... so by dialing in PA correction, you could then see a shift, as now the crosshairs cant move across the target, where as before they could...
hope that kind of makes sense.
In short, PA correction doesn't allow the reticule to wander across the image when it's dialled in. If your scope is not dialled in, you could see either inconsistent or wide groups, you could see a shift in POI but with a tight group (in which case your head is consistent but not aligned), or, if you are perfectly aligned and consistent you could see nothing unusual, but dialing out the PA error cures all.
It could be the reason why some very good shooters see shots wander in HFT, because getting that eye in the same place and aligned in all positions can be very difficult. Bobbing and weaving the head (assuming the stock/postion) allows you to determine if PA error is present to start with, and whether you are at the extremes of the crosshair movement or somewhat central.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep