When clear the system event log locally, the event details shown in CA shown the Domain\computername$ account in the WHO field.
The "System Event Log Cleared" event information comes from Microsoft Event Log record. In the native Event log, there is no origin information given. And thus, there is no information available for reporting.
This is an expected behavior in Windows 2003. The Domain\computername$ account is logged in WHO regardless who initiate the action.
In Windows 2008, there is functionality in the OS that allow to get the exact user account that initiate the action.