Lets get down to brass tacks here.
The fact is, there are those people who are gifted well beyond the rest of us and the rest of us look for the "secret" to doing what they can do and we can't find it. It can be an interesting, but pointless and eventually futile search. Cat Anderson was one of those "gifted" people.
Yes, these gifted players do work at their craft, but they have that little (or a lot) extra that makes choices much easier for them. Cat Anderson had the remarkable ability to relate his particular need into a mind numbing extreme of a mouthpiece that only HE could imagine, design, and play. Mortals need not apply. Cat did not use that tiny, shallow mouthpiece exclusively either. He used it when the part required it. (Probably most of the time, eh?)
Click on the Cat Anderson link for more info and a comment from someone who actually sat in the section with him.
I rather think that asking why something works for a player is akin to asking why Mark Twain, Charles Dickens, Shakespeare, Hemingway, and other great writers were able to write so much better than the average writer. Analyzing their writings is fine and fun, but it doesn't mean you'll ever be able to naturally write how they could nor will it give you any real insight to their ability.
We need to really be thankful Cat Anderson didn't use a huge and deep flugelhorn mouthpiece to produce that incredible high register. I can hear the crying and moaning from high note wannabe's now...