What happens is Windows stores the number of unread messages when you connect to an appropriate email program.
What can happen is user X signs on by user Y checks their email account. Then user X signs off and on again but windows still reports the number of user Y's emails, because user X then doesn't ever sign on to Y's mail it never updates the unread total.
It can also happen when you connect to your own email on a PC, don't read the emails then log on to another PC and read them on there.
Check this out if you want to have a clean up
http://www.pcmag.com/article2/0,1895,1936650,0 0.asp