Measuring of point not correspond to measuring in viewer

In last days, i have been taken measuring of hole pattern from a pallet, i use a plan line point alignment, before to start all measuring i check my cero, if is ok i start. After i had taken all points, i write nominal measuring (from drawing) for obtain measuring´s desviation, and in that moment i realize that the measuring is incorrect, i return to the position of my point and i review with the viewer what is the position of my hole, in the viewer the measuring showed is right. I take the point again and the result is wrong again. This mistake appear in many points, i took 35 holes position, and this mistake appear in 11 points
the mistake variation is rare, can be 8, 6, 2, or in this case (attached) is 19.1 mm aprox. anybody can help me?? this is stressing me

In the attached you can see a screen, where the viewer show the real measuring X:1549.785 Y:-499.889, and the nominal are X:1550 Y:-500, but in the result (remarked text) show X:1549.797 Y:-480.862 . So the mistake is in Y, and sometimes change to X, for this measuring Z is not important