The ability to export only 'new' or 'updated' items has clearly been added for use by the OU with xml export.
But this setting does not work for export to Excel or text files (and perhaps to Open Office too).
However the form used for specifying the export is the same for all exports and the 'Export new or updated grades only' appears for all export types and by implication should therefore work for all export types.
So clarification is required. Either the export for all types needs to conform to this setting or the description of the setting needs to have '(xml only)' added.