Hi,
Were you able to download an AdGroup Performance Report as Sreelakshmi suggested? if you have set the skipReportSummary header to false, then the downloaded report should have a totals row at the end which is the sum of all the rows in the report. Please note that when you compare AdWords API reports with the data available in the UI, you need to ensure that you have selected the same fields, filters and dateRange in both reports. Could you try this out and let me know if you are able to get the totals? If the AdWords API report doesn't match, please provide the report definition and the clientCustomerId via the Reply privately to author option so I can take a look.