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.