You may see addition hours worked by Volunteer for a Shift because the Salesforce Administrator at your org might have added Hours Worked while Assigning Volunteers to the Shift in Volunteers for Salesforce. This could have happened when you add Confirmed Volunteers from the Shift Calendar or from the Find Volunteers page in Salesforce.
See the screenshot below, if your volunteers are using V4S Kiosk or V4S Personal application to track their volunteer hours, please DO NOT enter any data in the "Hours Worked" field on the Find Volunteers screen. The "Hours Worked" field is automatically updated by V4S Mobile based on the hours actually worked by volunteers assigned to this Shift.
If hours are added to the Hours worked filed then even the actual Check-In and Check-out details get added to the hours in the Hours worked field and there will be different in total hours worked by Volunteers for a Shift.
Also, please be aware that when the Status on the Volunteer Hours record is changed from Confirmed to Completed, the Total Hours Worked field on the record is automatically updated by Volunteers for Salesforce.