It's suspected that this mutation arose from a single patient who had a long case of COVID-19. Here's an interesting pre-print study on an individual (not the one from this mutation) who harboured many mutations over a 101-day period. [https://www.cogconsortium.uk/news_item/persistent-sars-cov-2...]
In terms of this strain, the actual rate of mutation is higher (5.6E-4 nucleotide changes/site/year vs 5.3E-4) [https://virological.org/t/preliminary-genomic-characterisati...]