Hola,
Recomendacion: mirad el manual en ingles, que esta mas completo y actualizado. Mirad lo que pone
http://es.php.net/manual/en/ref.session.php :
Cita: session.gc_maxlifetime integer
session.gc_maxlifetime specifies the number of seconds after which data will be seen as 'garbage' and cleaned up.
Note: If you are using the default file-based session handler, your filesystem must keep track of access times (atime). Windows FAT does not so you will have to come up with another way to handle garbage collecting your session if you are stuck with a FAT filesystem or any other fs where atime tracking is not available.
Si tu servidor es windows con FAT, pues creo que no se eliminan los ficheros.
Saludos.