PHPExcel：重复进程会增加峰值内存使用量

I am using a "process class" witch performs some loads of Excel files, some getCell and setCell. Furthermore some writes to output.xlsx. The process class will also echo the peak memory usage. None of the input files changes during the process. Now when I run the process multiple times (e.g. because of PHPUnit) the peak memory usage will increase and I don't see why.

try{


$config = new ConfigDev();
if(file_exists($config->getOutputTmp())) {

    unlink($config->getOutputTmp());
}
$process = new ProcessRunner($config);
$process->run();
unset($config);
unset($process);


$config = new ConfigDev();
if(file_exists($config->getOutputTmp())) {

    unlink($config->getOutputTmp());
}
$process = new ProcessRunner($config);
$process->run();
unset($config);
unset($process);

$config = new ConfigDev();
if(file_exists($config->getOutputTmp())) {

    unlink($config->getOutputTmp());
}
$process = new ProcessRunner($config);
$process->run();
unset($config);
unset($process);


exit;

CMD echo:

\pathtoreporting\bin>php "My first Test.php"
10:19:58 Peak memory usage: 214.25 MB
10:20:23 Peak memory usage: 389.5 MB
10:20:51 Peak memory usage: 530.75 MB

At the end this means I cannot write tons of unit tests for my run method and this makes my code less safe.

Has anyone an idea why this can happen or how how to extract the part that where the high memory increases?

cheers, Peter