just a guess, but the next update should change to C.
The previous models released in December 2016 (announced September 2016), October 2019, and October 2021. So they would appear to be up for renewal in about a year, but no-one outside of apple really knows.
Guessing the Pros got one because to make the flagship AirPods match the flagship phones. And they needed to update the Pros fairly soon with the new chip for hi-res audio from the Vision Pro. So since they were going to have to re-tool the assembly line anyway, make sense to do both the updates at once.