On Fediverse Observer they used to show exactly the same number till several days ago. Regardless, look at the post/active user ratio on Lemmy on fedidb. It's 1200000/72000 or 16.6. For Kbin it's 44000/45000 or 0.97. It doesn't make sense the active users of Kbin to be 16 times less active than Lemmy's. To me that's evidence that whatever Kbin is reporting as active users is very different than what's reported by Lemmy. According to these stats Lemmy has about 27x the posts Kbin has. That number is probably correct since it doesn't depend on what's considered active. A post is a post. Yet Lemmy has only 1.6x the active users. That doesn't make sense.

If you use the total numbers for both from here, you get posts per user ratio of about 0.7 for both. That makes a lot more sense.