In Outlook 2016 (running on Office 365 mail), If I highlight the "inbox" folder, the 'Items' count shows 95 but the 'Unread' count shows 91. If I log into the OWA, the Inbox matches with the 91. I don't know what the 4 items are.
Also, I am unable to mark all as read and see what remains due to this being a shared mailbox.