In the Overview "Virus Detected" includes items listed on your "Incoming/Outgoing Mail Policies > Anti-Virus". The "Virus Types" are actual Viruses found within a mail.
Hence, Virus Detected on the Overview will also include Encrypted and Unscannable mails making to total larger.
Remember the Overview page has 2 seperate totals one for Inbound and one for Outbound. Virus Types contains a Total column.
Hope this helps.