The fact that one sets an expiration time does not mean that the keys will expire at that particular time. I'm not sure what is happening in the background, if there is a process like a garbage collector that expire keys, but some function do not activate the expiration check and return the key as valid, for example `getAllKeys` is not atomic and returns even expired keys.
$memcached = new Memcached();
//waiting more than 10 sec
$data = $memcached->getAllKeys();
var_dump($data); // key will still be listed
$key = $memcached->get('key'); // will trigger the expiration