Well, I can think of a possibility on a laptop, if it's on the manufacturer's default install - loads of laptop manufacturers will have their own "special" power profiles and will aggressively reset them if any changes are made. Yes, it's stupid, that's why I always do a wipe-clean install on a laptop.
On my desktop, I've got sleep set to 45 minutes on the 'balanced' plan. I can't remember it ever being reset back to the default 30.
Matthiasa's helpful "doing something wrong" comment - the main possibility IMO is a third party app messing with the settings. It seems unlikely that you would get corrupted registry settings on two machines. I might consider telling Windows to reset the power profiles to their defaults, then alter the 'balanced' plan as you see fit.
One other possibility, though it wouldn't account for auto setting changes - in Device Manager, check that the LAN/WLAN adapter aren't set to "allow this device to wake the computer". If your computer likes to resume for no apparent reason, that could be it.