Why is the total of metrics in a Report pulled via the Reporting API are different from reported in the UI Report with the same dimensions and date ranges?
Reporting API sums up the total of the line items which are not de-duplicated whereas the total in the UI Report are de-duplicated. It does not include the duplicate traffic data. It can be cross checked by adding up the sum of the line items in both the reports and then comparing them. You notice that API Report total get add up whereas the UI Report will not.