I, too, have often wondered if this was a regional difference or perhaps just a matter of habit and initial learning. I have, for decades, used the terminology of "Mercury-Argon" or sometimes even just "mercury" for shorthand if I knew that my audience understood.
This was most likely born out of how I first heard it referred to, however, it made sense as the Mercury becomes the dominant spectral trait in such a tube and the Argon is mostly just used as the carrier gas to get the ionization going and only contributes a small percentage to the overall UV output to excite the phosphors.
However, while the same would be true if Neon or another rare gas was used with Mercury, I instead refer to those as "Neon with Mercury" or "Krypton with Mercury" etc. I did, at one time, ask myself why exactly I would be inconsistent in this regard and I came to the conclusion that my reasoning for the latter examples is simply because it is not a standard and commonly used combination. Most neon filled units do not have mercury added to them unless you specifically want the color shifting effect from it. FWIW-- I actually do exactly this in many of my art pieces as I do like the effect, but beyond that, it is not a common practice. (do note that in cold climates, a mixture with a high percentage of Ne vs Ar is used as the neon will run hotter and help vaporize mercury for better output in phosphor coated tubes when outdoors in winter.....a 75/25 Ne/Ar mixture plus Hg being a common example, although I have seen straight neon used for this purpose too)
So, while that incsonsistency of nomenclature would normally be something that bothers me, I have, over the years, managed to forgive myself for it due to the above line of reasoning even if some may find it flawed.