Why is the number of opt outs shown in the opted out row of the Composition table different than the number of opt outs shown in the Opt Outs list?

The Advanced drop down menu in the Contacts section of your account provides access to all the opt out records stored for your account.

When you select Composition from the Advanced drop down menu, the number of opted out contacts shown reflects the number of currently stored contacts who are opted out.

When you select Opt Outs from the Advanced drop down menu,  the number of opted out contacts listed also includes any opted out contacts who may have since been deleted from your account as well as any opted out email addresses that may have been manually added to your account (as a "do not email" list for example) and are not currently stored as contacts.