This report is to compare older Fee Preview reports to the newer one in FeePreview. The results of the differences are outlined in FeePreview Compare. Simply download and load an older report as you did in the steps for FeePreview.
The FBA fees from FeePreview2 are used if, and only if, FBA fees cannot be found in FeePreview for the particular sku.  If the item is not found in either report you will see the error "Not in FP/FP2"