The calculation of %Difference on the Statistics and Counts report is the listing price to sold price whether 1 listing or many.
You can generate the Statistics & Counts report for one listing or many by clicking the Action Button in the Search tab.
To learn more about the Statistics & Count Report, please view the article HERE
Example: 1 Listing Result
Example: Several Listings Results (Please be advised that your report can include up to 200 listings from the search results.)
You can also add %ListingPrice as a filter from the Add a Field dropdown found at the bottom of the filter list.