Powerbook sleeps too soon


I've got a 12" Powerbook G4 1.5 ghz. I have the system sleep time for battery power set to 15 minutes, display sleep to 7 minutes, and HD sleep is at 10 (I don't think this is configurable though...)

When I leave my machine idle, the display turns off as it should at 7 minutes sharp, but the system goes to sleep around 9 mins and 45 seconds instead of the 15 the setting shows.

Does anyone know what could cause this?
I have the same "going to sleep too soon" problem with my powerbook. I have turned off the sleep on both the battery and power cord mode and it still goes to sleep in about 5 to 7 minutes.. only my computer doesnt arouse from its sleep after that... i have to shut it down and start it up again. this is really starting to become a niusance, does any one know what i can do to work this out??? thanks.