Zeroing the irons? It's been a few years but I feel like I rember zeroing irons then mounting the aimpoint and zeroing it. I think it is at least plausible that some refraction occurs through the lens. If you ditch the optic using the quick release and your zero was obtained with that refraction you no longer have a good zero. That's my guess. Or maybe it's an old photo from basic when they rated m4s but not the optic. That is the only iron sight I ever saw on an army m4.