I guess I would say "completely arbitrary" instead of "confusing" when describing fractional sizes. Since there aren't really any "standard" models for double basses, there isn't really a standard benchmark for what constitutes a particular size. There are plenty of basses that are considered 3/4 size that some people would describe as small, and others that seem quite large. Side by side, you would be hard pressed to call them the same size but they are both described as 3/4. The only time that I have seen any sense of consistency in fractional sizing is between different models offered by the same maker. If Shen offers 1/2, 5/8, 3/4, and 4/4 basses, I've got a pretty good idea which one is bigger than the other. If I'm comparing a Shen 3/4 to any other maker's 3/4, I have absolutely no idea which one is going to look bigger, which one is going to feel bigger to the player, and if those two distinctions correspond with one another.
Sometimes people suggest that string length plays a role, but that seems arbitrary as well. 40-42" could be described as 3/4, but the SuperKarr mentioned above has a 41.75" string length, and I would definitely consider it a 4/4 instead of a 3/4. Joel Quarrington's Maggini has a string length around 39" and very slender ribs, but I would also not consider it a 5/8 instrument.
With instruments that are not by modern makers offering a range of sizes, the distinction tends to be mostly based on the shop/owner's perception of the instrument. If it's smaller than most of the other basses in the shop, then it's probably considered 5/8. If it's larger than most of the other basses in the shop, it's likely called 7/8 or 4/4. I have even seen basses described as 9/8 or 5/4 before and while they have definitely been big, I wasn't entirely convinced that was necessary. If a bass doesn't fit into the smaller than most or bigger than most categories, it's typically lumped into 3/4.
I would be curious about Mike Shank's decision to consider your bass 7/8 instead of 3/4. It would also be interesting to see if it went to another shop if they would make the same distinction.
My experience with Upton basses is limited to what was in the shop the day I went for a visit, which by no means qualifies me to make generalizations about their output or sizing decisions. The SuperKarr was definitely large, but that was also very much intended and the purpose of that build. I also had a chance to play the Concord prototype, which again was designed to be their Prescott tribute and as such is a large model. The 3/4 size basses they had in the shop didn't strike me as atypically large, but I also play an a bass with quite wide/round shoulders that I would describe as 3/4 but feels quite large to play. I am sure the Sheriff has much more knowledge about them than I do, and if you are considering commissioning a bass from them, they will likely want to talk about dimensions like string length, widths, and lengths other than just 5/8 vs. 3/4.