The answer is simply because I'm BIASED!!!
Actually, it's a basic question...as basic as simple geometry.
The bunkers at Aronimink are almost all of basic geometric form in shape, with all of them just flat sand bottoms with grass faces. THe outside shaping is rectangular in most cases, similar to something by Seth Raynor.
The bunkers at Merion were much more free form, with all sorts of capes, bays, twists, turns, and sand flashed up to the top lip at bunker height. They had internal contour within the bunkers themselves. 80-90% of them had more twists and turns than anything one finds at Aronimink.
Given the differences in complexity, the bunkers at Aronimink lent themselves well enough to the machine-shaping, modus operandi of MacDonald and Sons. The bunkers at Merion did not.
The same solution applied to two different questions leaves us with the questionable results. Simple as that.
Part of the equation was also almost certainly due to the fact that Ron Prichard seems to me to be the kind of guy who would be onsite as much as possible and puts his heart and soul into "restoration", as he is a true believer. I assume that he'd work hard to try to get the details right. In the case of Merion, I'm not sure that Tom Fazio could make similar claims.