A good example of the problem is seen when changing from a low ribbed gun to one with a high rib. If that little bit of rib is visible peripherally or subconsciously at the moment of firing, he knows that his elevation will be as intended. One time he tried a Citori Sporting Clays gun with an arched rib that was elevated at the rear and low at the muzzle. He found this to be the wrong subconscious sight picture and was unsettled On the other hand, the gun's owner shot the arched rib setup quite well and claimed that he could not get his head down properly on the Technoid's gun. Both guns shot to the same point of impact, so the only difference was in the sight picture.
Some people are also fussy as to rib width.
If the rib is too narrow it makes them aim. If too wide, it may does not give them the precision they require. Visual tricks can come into play here. Churchill used a rib tapering from broad to narrow to give his short barreled guns the illusion of more length. It worked for some people.