So just now my PM count was at 99, I refreshed every so often, staying on the main page and watching the number. Eventually I decided to check the actual PM page and saw that I had a new PM even though the counter was still at 99. I clicked through to the new message but noted that the counter was still at 99. I went back to the main page then back to the message, still at 99. I then replied to the PM and checked the number again, which is now at 101.
So it looks like the number only updates when you respond to a message but not when receiving one.
EDIT: Just actually tested it. I received a message and my count stayed at 101. Then the person who replied edited the post and it's still at 101.
So it looks like the number only updates when you respond to a message but not when receiving one.
EDIT: Just actually tested it. I received a message and my count stayed at 101. Then the person who replied edited the post and it's still at 101.