Hmmm...I do see your point.
Session.SessionID will be unique but only until the server is restarted.
Okay...keep doing what you are doing.
But consider passing your DB-generated session id along via means other than (or in addition to!) session variables.
An optimist sees the glass as half full.
A pessimist sees the glass as half empty.
A realist drinks it no matter how much there is.