X509 Certificates

When I select properties on a selected user ,in Active Directory Users and Computers (in advanced view), and click the tab Published Certificates some users seem to have one or more published certificates. We are using smartcards to log on to the domain, so I thought that this were these certificates.. But some of the users do not have a published X509 certificate even if they use smartcards to log on to the domain !! So my question is; can someone tell me what these certificates are ? Why do some users have these certificates while others don't (even if all use smartcards..) ?