@Sampei yeah good catch. Measuring accounts on the fediverse is really hard. The 11M comes from a different source than the 8.9m figure you use. I wrote about all the different sources when Mastodon had 10M accounts: https://fediversereport.com/10-million-mastodon-accounts/
It's really hard to know for sure which source is correct. My gut feeling is that 11M is too high. But the 11M is at least the consistent same source as used for the 10M one, so the +1M masto accounts in 25 days is reasonably correctish