You're talking about on the left panel right? Reason being is it refreshes independently of the rest of webmail.
A little above "Inbox" you'll see "refresh folder list" click that and if you have nothing unread the number should go away.
I haven't tried this feature yet but this may work for you. If you go to Options, Display Preferences there are settings to force refresh of the folder list.
Again I haven't tried that feature but it may be worth doing if it's bugging you.