I ve just seen in the PHPExcel Code, that the activeSheetIndex in the PHPExcel class is not updated in case you create a new worksheet using createWorksheet() with no index parameter. The new Worksheet is just appended, but different from the else-branch
where the index parameter is given, the activeSheetIndex is not increased.
Don't know if this is a feature, just wanted to let you know.
Best regards, Chris