I presume you're talking about accounting for VPN connections, correct?
If so, then at the moment the PIX doesn't do this. The next release (v7) coming out later this year will, but unitl then it's not supported, sorry.
The entries in the Passed Authen/Failed Attempts log are simply from the authentication requests. For someone to appear in the Logged-In User list on ACS they have to have accounting enabled for that authentication.