On Instagram, when you make a new post, you get the option to share that post to Threads (the Twitter-like social media related to Instagram). This can be done if you have a Threads profile connected to your Instagram profile, and Instagram lets you create one very easily. You even have the option to share ALL posts you ever make automatically, without even being prompted about it again. This means that a portion of the posts on Threads are posted by people who didn't post the posts on Threads, but instead posted them from Instagram, without touching any of the Threads' interface or even looking at it. Worse yet, some people (like me), don't even USE Threads AT ALL, but we're still able to post things there by clicking the setting on Instagram.
Don't blame me for this. Just look at it from my perspective. If I can get even a single extra view on my posts for free, why wouldn't I? Why wouldn't you just share everything you post on your Instagram account to your Threads account? It's like free exposure.
I do wonder how does this reflect on Threads' metrics, though. For example, let's imagine 90% of the posts on Threads just came from Instagram, and Threads said "we have 1 million posts per day!" The reality would be that only 100 thousand posts are being made by users who actually use Threads, and the rest is just coming from Instagram. Similarly, what if they count a Daily Active User (DAU) as a user who has made a post on a given day? That would mean people who never even see Threads' interface would be counted as users. Of course, this is just me speculating, I haven't bothered to look at how they count things.
Another interesting thing is how does this look from the actual Threads user perspective. If they see a bunch of people cross-posting from Instagram, do they care? Many users are lurkers who never post or comment on anything. They're passive consumers of entertainment. They use social media like a television. I don't think they would care very much if the user was actually active on Threads or not. They're getting the content, and that's all they want. At this point, Threads is like an RSS client for them. It's just a different interface for the same stuff they would get on Instagram.