It depends on the version of Lemmy that your instance is running. My account is on sh.itjust.works which upgraded Lemmy versions a few months ago. Apparently it changed a parameter name that keeps track if a post has been read or not.
So on Sync, you can click on posts, but they never show as read. It’s super annoying, so I went back to Voyager.
That’s not an impressive number. That’s like 2 days’ worth of driving.