Apparently, some members who were active and had posts have had their member account accidentally removed. This should not have happened. Their old posts will still be there and can be found in a search for the user name, but the account doesn't work. It only happened to those who had not logged in for 6 months or longer.
To fix this, please do the following.
1. Create a new account with either the same or a different user name
2. Send a private message to me giving me the original account name.
3. I will re-attribute the posts to your new account name. Your post count will be updated to show the correct quantity.
If your old user account had no posts, just recreate it.