A static list is a snapshot of the exact prospects (by eTrigue ID number) that matched your criteria at the time the list was saved.
When opening a static list in the List Builder, the Results pane shows the original criteria of the list and the number of prospects that matched your criteria at the time the list was saved. This is done by design to provide administrators with a reference to the state of the original list as it was when it was created.
If any of the original associated prospects have been deleted or merged into other prospect records, those prospects will not be shown on the Prospect Results page (because the specific corresponding records no longer exist in the DemandCenter database). Similarly, when a static list is in use by the system (such as when sending proofs, testing Email Actions, or running campaigns), the system will only include prospects that still exist in your database.