Because some of us grew up when "No Shift In Zero" when removing and re-installing QD mounts meant you were lucky if POI shift stayed in the same zip code
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MOE handguards on one upper and standard plastic ones on the other, but I zeroed resting the mag on the bench to avoid any POI shift from a rest touching the handguards.
Before trying the Aimpoint on the second upper I would have guessed it'd be within about 2-3 MOA, but I didn't really have any basis for that estimate. That's why I was a bit surprised the zeros were so far off.