TimedLoginDelay seems to fail to take smaller values then 30

Binary package hint: gdm

in ltsp kiosk mode we use the following setup for gdm.conf:

[daemon]
# Automatic login, if true the first local screen will automatically logged in
# as user as set with AutomaticLogin key.
AutomaticLoginEnable=true
AutomaticLogin=kiosk

# Timed login, useful for kiosks. Log in a certain user after a certain amount
# of time.
TimedLoginEnable=true
TimedLogin=kiosk
TimedLoginDelay=10

apparently the initial login works fine and ~/.xsession (starting devilspie, metacity and firefox) of the user kiosk is initiated.
if the firefox window is closed the user is dropped back to the login screen as expected, but there a timer with a minimum 30 sec is running until the next automatic login is initiated, no matter what value is set for TimedLoginDelay.