This can be caused by different holidays around the world. Consumers have more v6 adoption than companies

There is a lot of jitter in the data, which makes projections using just a few days or even a month unreliable. Looking at 2020 through present, it looks somewhat like draped curtains with peaks around Christmas/New Years and July/August, usually followed by a bit of a slide down, but the year-over-year trend appears to be upward.

My working hypothesis is that businesses and schools are less likely to have IPv6 than home ISPs so when employees and students are on vacation (around Christmas & New Years, and in July and August) they are less likely to access Google services from their employers or school and more likely to access those services at home, swinging the ratio towards higher IPv6 usage. Same thing with weekends off for most employees and students.