You edit a group membership report and select the fields you want on the CSV options page. You then configured the report to expand nested groups inline and export to CSV, however the following additional fields are in the output which were not shown on the CSV Options page:
The additional fields are hidden fields used in the report layout to properly display the inline group expansion.
When exporting to CSV, select to expand the nested groups in a subreport
Enhancement request VSTS003500 has been submitted to Development for consideration in a future release of Enterprise Reporter.