Its very carrier dependant for how much gets invested in infrastructure, but in the US carriers seem to be updating frequency bands almost every generation model of phone.

Go look at a few models of the same phone for instance pixel 6 vs 7 vs 8 vs 9. Carriers are phasing out antiquated cellular bands. They try to keep the most widely used bands like n71 or other most common 4 or 5 bands but then tweak or change another 10 bands inside the phone sometimes 20 bands in all.

But I assure you this is the case. Cellular bands change as towers get serviced and replaced.

*source phone repair tech for the past few years.