Just how big is that driver?

I have noticed the JM lab drivers are fairly accurate if you account tor the surround. If you add the frame then it doesn't work out so neatly.
The hole in the cabinet may only accomodate the basket spider and magnet assemblies with the whole shebang held securely in place by virtue of a mounting ring which must rest on the baffle. Since this is very variable from one manufacturer to another I would not consider it a meaningful measure.
It is generally not helpful at all to concern oneself with the size of the driver. I do not know pro applications and it may be relevent in big spaces. My 15 inch woofered speakers do not have the bass that my double 8 inch speakers does so again it boils down to a great many parameters working in concert. Uless you want to argue that 2 x 8 = 16" but in all seriousness the reactivity the resistance to movement as well as piston speed = magnet and voice coils along with cabinet geometry all make a big difference.
Its not important when considering a bass transducer. My current drivers are speced to a low of 30 cycles but I lived quite happily with others that made no claim below 50 and were falling fast below 80. Because that is where the music- my music - anyway lives. Save the rumble for HT please.

Gawdbless

Is this a 'never mind the quality, feel the width' sort of question?

No.

I’m in fact wondering where the truth is for one, and why such disparity exists in this ‘plain to see’ facet of the audio industry.

One can argue much about claimed parameters of just about any audio component. Are tube watts indeed superior to solid state watts? What’s the true maximum output of a 300B tube? How was that SPL number arrived at? …. And likewise, impedances, and sensitivity?

But how on earth can any despairity, difference, in critical or even subjective terms, alter a simple ruled result? It’s misleading and undermines consumer confidences. IMO

On one hand, I feel the driver has to be considered the whole of the active sound producing unit. The cage, voice coil, surround, magnet, vibrating element or diaphragm… the whole enchilada… the part that fills the hole in the cabinet.

On the other though, with such often made references to the type of cone materials being used, paper, reinforced this & that’s, Kevlar, titainium, aluminum, coated such & such alloys; it’s only that portion moving the air itself. In short, the part we can actually see working.

Years ago, when one speaker maker said they used a six inch midrange driver, you could count on it being six inches across the paper cone itself. I see that no longer being so much the standard and simply wondered “how come dat is?”

I must have missed that memo.

Just a guess here, but, I wonder if some round up or down drivers made to metric proportions.

Unsound...The world is metric...except for the USA and Fiji

Exactly, and many drivers are made outside of the USA. So as long as US manufactures feel the need to simplify things for their US customers, there a chance that the integrity of specs might be compromised.