I've been asked so to explain... The aperture is slightly larger at .230 large/.100 small for a larger field of view vs a normal A2 sight at .200 large/.065 small which is kinda narrow.
The two main things for me is 1) it allows you to go from large to small without shifting your zero and why I prefer them. Otherwise there's an offset/elevation change when you go between large and small. 2) The other is the small aperture has a concave/dished out surface around it for gathering more light only on the A2 they put it on the wrong side facing towards you. It should be away from you so you get a solid defined ring when you look through it instead of a fuzzy one when it's facing you. Those two things have always bugged the crap out of me with regular rear sights on AR's.