Outlook 2000 – opened/unopened messages

My boss keeps thousands of e-mail messages. (“just in case”) I have been asking him to please clean them out. During his cleaning process he noticed the number of unopened messages posted on the folders list, not coinesiding with the actual number of unopened e-mail. (when he would sort by unopened messages)The real strange thing is that these “upopened messages” were old e-mail that he had opened at one time. Why?