Like, the differences in the 1991 basket to the 1992 basket are pretty small (please, someone, double check this). But the differences in the 1991 basket to the 2019 basket are larger, for example.
So it would seem to me to be a bit silly to have to update the prices of the 1991 basket all the way to today, especially when you're updating the 1992 basket as well, and every other year.
I'm not a CPI expert person, so if there is someone that really does understand these things in depth, please chime in!