PHP Session *not your usual* problem

[ysamu]

Hello,
I have a funny problem with my session management, actually I never came accross this before.
I have a website, an admin panel which is protected by a session object. I have, I think "standard" PHP session settings and my problem is that my system never logs me out after 20 minutes of inactivity.
My code for admin access is:
----------------------
if(isset($_SESSION['user']))
//he is loggedin, carry on
else
//send to login page, set the session on sucessfull authentification
----------------------

My php session settings are:
session.save_handler = files
session.use_cookies = 1
session.use_only_cookies = 1
session.name = ds
session.auto_start = 0
session.cookie_lifetime = 0
session.cookie_path = /
session.cookie_domain =
session.serialize_handler = php
session.gc_probability = 1
session.gc_divisor = 100
session.gc_maxlifetime = 1200
session.bug_compat_42 = 0
session.bug_compat_warn = 1
session.referer_check =
session.entropy_length = 0
session.entropy_file =
session.cache_limiter = nocache
session.cache_expire = 180

According to those, I assume the garbage collector will have a 1% chance deleting any session after 1200 seconds.
There is enough activity on the server to trigger the GC often, but I stay logged-in for weeks!

Would you know what am I doing wrong?

Thank you for your help!