It depends upon whether the shot is taken consciously or subconsciously. If you have time, or feel that you have time and can consciously think through the shot, sure one should be able to visualize what "13in" low would be (though I have no idea why one would set a "zero" as such). However, if the shot is rushed subconsciously we want to align the object of our desires, with the only reference that we have... The crosshairs. Throw in the fact that we also tend to want to aim a "little high" because it's looks far, and you get lots of high shots.