Those numbers are never really accurate. Depends on how you are handling stale accounts, spam accounts and how well you have managed to keep spammers in check over the years. The actual registrations by user ID is over 2 million here.
On my own forum I have just over 9K members BUT spammer accounts and stale 0 post accounts are no older than 2 years. Both the post count and user count are fairly accurate for real posts and members that have actually posted something. I have 77 posts per user, it's only ten here.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison