After an email address has volunteered and confirmed once for an Organization, any subsequent sign-ups from that email address from the same browser will be auto-confirmed.  That “Confirmation Required Once” behavior is the default Confirmation Mode.  If you prefer for volunteers (outside of Kiosk Mode) to always receive the confirmation email that they must confirm, you can change the Confirmation Mode to “Confirmation Always Required.” This Confirmation Mode setting is under the Settings tab | Emails sub-tab. Your choices are: “Confirmation Required Once,” “Confirmation Always Required,” and “Confirmation Never Required.”
NOTE: Using “Confirmation Never Required” is not recommended for normal use.