Since your code was generated by the data assist wizard, the detail page was alredy set to filter the recordset.
In data assist pages, the detail page will loaded in three conditions:
1) from a link on the Results page
2) After Inserting a record.
3) After updating a record.
The Querystring variables are used when going from the results page.
The session variables are created to hold the id of the updated or inserted record to filter the correct record after insert or update.
In Jeff's case, he only needs to filter the record based on the query string variable since the results page he is creating will not be loaded after insert or update.