If you filter your sessions results by site, there is a chance that you will see a different number of sessions.
DSS mandates that sessions are reported against the Case site.
When you search for sessions in the DSS Export tab, the site filter will always use the site saved against the Case.
When you search for sessions in the basic or advanced search tabs, the site filter will always use the site saved against the session.
The site saved against a session is (in most cases) not editable and is populated by the Site the user has logged into.