For those who don’t know: The way Masotodon federation works, your server only knows about other servers that its members follow folks on (and I think there is some further scoping-down from there). So if you see a viral post, that may have hundreds of responses from folks on small Mastodon instances that don’t peer with yours, so you can’t see them. But the author of the post can!
It’s like hosting a Q&A session in a big auditorium with 1,000 people where many people think it’s a crowd of 10.