You are seeing the difference in Hours Worked for a specific Shift because you may have an entry in the 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 attached screenshot. If your volunteers are using V4S Kiosk or V4S Personal to track their volunteer hours, please DO NOT enter any data in the "Hours Worked" field on this screen. The "Hours Worked" field is automatically updated by V4S Mobile based on the hours actually worked by the various volunteers assigned to this Shift. If you key in some data into this field while assigning volunteers, the auto update of "Hours Worked" based on the actual hours worked will add the actual hours worked to this field. So you will see a difference between the actual hours worked on the Shift and the number that you see in the Hours Worked field for your volunteers.
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.