Dates are Australian 3am.
Archived threads that aren't featured are eventually erased along with their data.
My guess is that the drop from yesterday occurred after the other Admin deleted the vestigial and hidden /tech/ board. Another possibility is the deletion of threads in the archive, which occurs automatically 50 days after a thread enters the archive.
I do not know why the number of posts would be affected. Perhaps strategic thread deletion in the archive caused it, or perhaps it was caused by tinkering with the statistics feature yesterday by the Admin-Dev. Perhaps a developer has a better answer.