I'm not associated with AVS, but I can think if two reasons why their dimensions might be exactly the same:
First, maybe those dimensions make very efficient use of board space and assembly jigs and workers who have it down pat.
Second, maybe the PE box was very thoughtfully designed as far as internal volume and internal dimension ratios, and/or maybe the VSA version has slightly different internal volume and internal dimensions that are ideal for their design, and it just so happened that fit like a glove in the Parts Express box's dimensions.
A third reason might be that there's no need to redesign the shipping materials.
That said, I don't buy for one second that Bertie Von's cabinets started out as different stock models than the PE cabinets, simply because of the baffle attachment and their use of the same horribly deleterious grille.
Seriously, an expensive speaker should have a less risibly-engineered grille.
Hi Vapor
Are you kidding me right?
what about filling material that dampens these things
anyway most midrange and tweeters are back shielded
for
cabinet resonance you put reinforced frames etc.
I disagree with you... 
There's nothing to disagree about, because it's a simple issue of "right" vs. "wrong." Here, you are the latter. Good use of damping material (which is a lot more than merely lining the walls) does help a lot with standing waves and basically kills back wall reflections, but it does nothing to address, for example, edge diffraction. Also, mere reinforcement is often not useful to deal with "cabinet resonance." Too much stiffness makes things worse, in fact. Read through the Harwood BBC papers on cabinet design. A smarter approach is the one outlined by Duke above: "strategically-placed constrained layer damping." See also the propaganda ("white papers") for the KEF LS50 or the earlier iteration of that approach, Tannoy "Differential Materials Technology."
