The Associate & Cashier performance reports, common to retailers who use the Check In/Out feature, has a flaw in it’s default design.
By default under OPTIONS (when you edit the report) the “Receipt Types Allowed” field is set to look at NORMAL and CANCELLED receipts. It should NOT be looking at cancelled receipts. Cancelled receipts happen when someone Cancels a check in/out due to a missed check in/out.
If you run into a client complaining that the Check In/Out: Shift Report is showing different hours than the Performance reports (and you’ve determined the problem is NOT with the SHIFTs defined) this is most likely the cause.