I think what he is talking about, and what bothers me about SCAR sights, is that if you have to move the post tip out of the center of the circle to get zeroed, it can throw off your shots.
When I look through the SCAR sights, my mind automatically aligns the front hood with the rear aperture. This would be great if that happened to correspond with the correct zero. However, at least on my brother's SCAR 16, to get it zeroed at 50 yards, the front post tip is noticeably lower in the hood. If I use the front post tip centered in the rear sight, I am dead on. If I go off my 1st instinct and "circle the circle" I hit really low on the target. I don't do this on standard AR sights because the front sight is an open "U", I automatically put the post tip in the center of the circle since there is no other circle to align that my brain likes better than a circle and post.
There may be a way to bring the rear sight up without changing the BDC and then re-centering the front sight post in the hood, but I don't know for sure and he gets mad if I take his rifles apart.