They had different models with different capabilities. As they made minor style changes, they bumped the numbers slightly. The 81–82–83–84 were basically the same concept, as were the 85–86. The 89 and 92 were higher-end models. The 80 and 73 are simpler models intended for middle school.

All of them are basically a multi-generational scam perpetrated against the hapless parents of American high school students who were told that they needed to buy overpriced anachronistic calculators for their kids to succeed in school. In my opinion the calculators have overall caused more pedagogical harm than benefit; the students would be better served by some combination of (a) problems that can be solved without the tedious but trivial numerical calculations these calculators support, or (b) are solved using a real programming language. If someone really wants to assign simple numerical problems, give the kids slide rules.

Calculators of this type used to make sense for an engineer doing work in the field somewhere, but make no sense in the context of a classroom.

One other factor that others haven't yet covered is that the different lines had different capabilities, e.g. the T-89 had Computer Algebra System symbolic manipulation meaning it could pretty much solve many types of math problems on its own, so it wasn't generally allowed in school. And then the Ti-85/86 was a step down, but had matrix support that the lower models lacked, so it was necessary for some specific types of classes.

My favorite was always the TI-85/86 line. I loved those F1-F5 buttons right beneath the screen, which made the interface overall better to navigate. The first programming I ever did was on one of those (either the 85 or 82, can't exactly remember at this point which I owned first). And, the only thing of note I ever had stolen from me was a TI-82, taken out of my unattended backpack by another student during gym class :( (And I had even carved my name into the back of it with a knife, so it would've been identifiable.)

They were different lines. The numbers aren't mean to be chronological; similar to how AMD released some 5000 series AM4 Ryzen chips long after they'd moved on to AM5 and 7000/9000 series.

TI83 (1996) was a successor to the TI82 (1993) which was a refresh/update of the TI81 (1990).

TI85 (1992) was the second model they made, originally intended as a higher end version of the TI81.

Similar reasoning for the rest of their line up. Different models had different features, and then those models would get incremental updates/refreshes over the years.

I wasn't part of the team or anything, so if anyone has any insight to why exactly they called it that in the first place, I'd be interested to know, but generally speaking the answer is: When they released the first one in 1990, they didn't name it under the presumption that this family of devices would be a staple educational/academic electronics device for the next 3 decades with dozen(s?) of different iterations/generations over the years.

A lot of it had to do with capability. The TI 92 was considerably more capable than the 83. The 89 had better software than the 92 but with a smaller form factor. The 92+ was the 92 with the 89 software.