Before a true comparison can be made, the test conditions have to be exactly the same. The stand, the gun's firing position and the shooting angle all have to be same, for example, to ensure that the reason for different target locations is not something other than the projectile's flight path.
One reason may be that the velocity is not the same. All manufacturers measure the pressure and velocity of barrels that correspond virtually to the minimum dimensions both for chambers and barrels. This is done for safety reasons. Gunsmiths also try to prevent problems with excessive pressure and take certain plus tolerances into account in their dimensions. If these dimensions are greater than the ammunition manufacturers', there will be reductions in pressure and velocity.
Short or worn barrels naturally also result in lower velocity. In all probability there are bullet manufacturers who determine their ballistic coefficients with calculations and not with measurements after firing shots. If the BC used for calculations is too good (e.g. 0.45 instead 0.38), the projectile flight path obtained will be straighter than it actually is.