This is a really interesting discussion....
If we're discussing geometry the answer is very simple: the pipe is geometrically flawed - we assume the cross section of the bowl on the classical billard, at any given point of it's height, should be a circle. That implies symmetry between the sides and between the front and the back of the bowl. If there's a notch on the back, and the front goes on with it's curve, then the cross section at that point becomes oval, thus making the pipe geometrically imperfect. But... who cares?
Why do we assume an artisan like Jeff would want to make a perfectly perfect pipe without giving it a personal twist? Wouldn't that be boring? Is there a written certificate from the maker that says "Classical Billiard"? Or is this a fairly classical shape made by that particular artisan? Because there's a difference and I think the latter statement is closer to the truth.
There are so many pipes out there, made by some very skilled artisans, that present really weird (and sometimes beautiful) stylistical choices. The point is that if you know the pipe has perfect fit and finish, the wood and stem material are absolutely highest quality, then the only question is "do I like it or not?". Toku makes masterful assymetries that when first looked upon seem really weird. Ingo Garbe makes theoretically cylindrical shanks that become oval towards the bowl and they look spectacular, even though geometrically imperfect. Jeff makes an undercut above the bowl/shank junction, and you can like it or not, but the fact that it's a detour from the geometrical description of the prototipe of that shape is quite irrelevant.
What I want to say is that in our times, when there are so many skilled artisans, once one has reached a level of craftsmanship that allows him to create pretty much anything he can imagine, there's really no need to play by the rules. If you can make a 16 side facet shank, but you just want one facet to be thinner than the others "just because" (or in a more private discussion: "because f*** you, that's why"

), well, it's just a matter of the buyer accepting (or not) your choice. If he doesn't he will have to settle for someone else's pipe, because you do it that way.
The value of artisan pipes is based on their uniqueness and the personal touch of the maker, definitely not on their geometric precision- and that's great, otherwise we would be seeing only really boring exercises in lathe turning instead of beautiful pipes.
Sorry if what I wrote is unclear, it's actually quite clear in my head but I'm not educated in art critique so I'm not very good at explaining it...
All of this is just my personal opinion and anyone is free to disagree
EDIT: Well, since it turns out there's really no undercut in the discussed pipes, this whole banter doesn't really make sense...

but still, it was worth dedicating some thought to it.