10.4/10= 4% off
If youre only within +/- 3 meters then that could be 3 of the 4% its off.
Id also be suspicious of your holding fixture moving as well.
And youre saying you looked through the scope, noted the location on the reticle, walked down there to the target, made the mark, walked back and then your reticle didnt match where you marked the target? I think shooting a group, adjusting 10 mils and then shoot another and judge by where youre group centers are, its more reliable than trying to transpose your reticle to a target 100 meters away from you.
Not saying youre wrong, just saying there a lot of issues in what your methodology could or could not be from the info provided to us.
And you have a collimator?
The exact distance isent really what matters in this case, also i think you got it wrong how i did it. Here is how i did it:
1: I set up a gridpaper at 100 m ( using a range finder ).
2: I click the 10 mil subtension mark ( the hold over mark thing ) so it matches an exact spot on the grid paper.
3: I walk down to the paper, and put a little red round sticker where the 10 mil holdover mark was at for better visual.
4: I walk back - check and triple check that the 10 mil holdover mark is now exactly at the red sticker.
5: i click 10 mils, that center crosshair should now be at the mark where the 10 mil holdover was at, but it is not.
6: I click 10 mils back, and the 10 mil holdover mark is again at the spot i marked.
I did that 10 times or so.
No, i dont have a proffesional coliminator like the factories have, but i have one of those you put at the end of the barrel. If clicked the 10 mil holdover mark over a spot in the grid, and tried to dial 10 mil. Same story.
That means, that either the holdover marks are not true, or it dosent track, and i am very sure its the tracking.