It’s based on a sample and extrapolated out so it’s entirely possible initial immunity was overestimated. Possibility for bias in the sampling of who was tested and such. Hopefully it’s just a statistical fluke but everyone is sort of on edge at this point watching for this starting to happen.
It’s not really a question of if the virus will mutate to reinfect people again (or infect those vaccinated) but more of when and if we can get to herd immunity before that happens.