listWorksheetNames() is giving incorrect result

$objReader = PHPExcel_IOFac开发者_如何学Gotory::createReader('Excel2007');
$worksheet_names = $objReader->listWorksheetNames($input_file);
print_r($worksheet_names);

some times it is printing

Array
(
    [0] => test0
)

some times it is printing Array ( [0] => test0 [1] => test1 [2] =>test2 )

there are total 3 sheets in my excel file..test0, test1, test2

any idea why it is happening?