We recently had a support ticket asking to explain why an opportunity on a school's opportunity list page showed a number of volunteers matched to that opportunity when no notification had been sent to school volunteers.
There are three possible reasons for this -
First, if you use the EZVKiosk to sign-in volunteers at your school and if the opportunity is configured to DISPLAY KIOSK then whenever a volunteer selects that opportunity on the kiosk they are automatically signed-up (MATCHED) to it. This ensures that you can track all the volunteers and hours associated with that activity at your school.
Second, if your opportunity is configured with AVAILABLE ALL any volunteer at your school can see the opportunity when they sign-in to their EZVolunteer account and can sign-up for the opportunity from there.
Third, if your opportunity is configured to display on your calendar page any volunteer can sign-up when they view the details of the opportunity on your calendar.
If you want to limit matched volunteers to only those who receive and sign-up through a notification email, do not configure your opportunity with any of the above settings and it will only be available for sign-up when you use the NOTIFY OPPORTUNITY action from your volunteer list page.