Session Class - Wiered Behavior

[eluser]Twisted1919[/eluser]
Hi , i am getting some wiered behavior from the session library .
I am storing the session data into database and , my config looks like this :

Code:

$config['sess_cookie_name']    = 'session';
$config['sess_expiration']    = 7200;
$config['sess_encrypt_cookie']    = TRUE;
$config['sess_use_database']    = TRUE;
$config['sess_table_name']    = 'sessions';
$config['sess_match_ip']    = TRUE;
$config['sess_match_useragent']    = TRUE;
$config['sess_time_to_update']     = 3600;

Right now in my sessions tables i have :

Code:

SELECT *
FROM `sessions`
LIMIT 0 , 30
//Result :
Showing rows 0 - 29 (3,690 total, Query took 0.0004 sec)

My problem is that , it logs me out after 10 minutes of inactivity , sometime less than 10 minutes even if the time is set higher than 10 minutes ...

Why that ?