Session does not expire? why?

[eluser]mindprojects[/eluser]
Hi to all!,
my web applicaiton is running on CI 1.6.3.
This is my session config:

Code:

$config['sess_cookie_name']        = 'ci_session';
$config['sess_expiration']        = 0;
$config['sess_encrypt_cookie']    = TRUE;
$config['sess_use_database']    = TRUE;
$config['sess_table_name']        = 'bk.ci_sessions';
$config['sess_match_ip']        = FALSE;
$config['sess_match_useragent']    = TRUE;
$config['sess_time_to_update']         = 5;

Following the idea that,if i'm closing and opening the browser in a range of time more than 5 seconds(sess_time_to_update),session should expire,shouldn't?
It doesn't happen,everytime i'm getting session data always on. Why?

Thanks in advance.