For the record, somebody provided an alternate explanation along the lines of 'they are doing a dumb thing'. Specifically, population estimates are less reliable than vaccination counts, and as vaccination rates go near 90% and over, the unreliability greatly increases when computing unvaccinated_estimated = pop_estimate - vaccinated. A corrected graph using a different population estimate:

https://twitter.com/JamesWard73/status/1436017485262823424?s...