A lot of accounts are interacting (voting, posting, etc.) on lemmy-visible activitypub services within a 6 month timespan, but most accounts are not active users interacting every month.

It's actually a very positive graph. Many of the new accounts would be spammers, bots, throwaway accounts, alts of banned users, users making account on multiple instances because of downtime, etc. So it's normal to see growth over longer spans of time that aren't completely reflected in monthly active user statistics.

The current plateau is probably for the best, it gives developers time to catch up somewhat with the last growth spurt. There will be other social media platform clusterfucks in the future that will kick off future growth spurts.

My guess: monthly user add all the unique users in the last month. Half year add the unuque users in the past 6 months. This is why is bigger, but it grows slowly in the months with less unique users.

I'm guessing unique users who have been active on Lemmy within the past 6 months.