Searching email messages on an iOS device may cause the unread message count to increase. Existing messages go not get marked as unread, only the ticker count changes. Unread ticker count does not change in Outlook or OWA.
In Outlook, mark the entire inbox as unread. Wait up to two minutes for the iOS email client to update to show all email as unread. Then, mark all email as read. As the iOS device synchronizes, it should update and reflect the correct number.