There's idle, there's idle running halt-on-idle and then there's load. Sitting in bios doing nothing is not the same as sitting at the desktop, while running halt-on-idle, doing nothing. You'll see the same effect just sitting at a Dos prompt, or sitting at the dialog window, where your O/S is waiting for you to input your password. For my setup, I may be getting 21C during halt-on-idle, 33C during simple idle and 37C under full load.
Yeah, i just learned that today. While sitting in bios watching the pc-health. cpu temp climbs to 40°C, then while sitting on the desktop its at 28°C, and at full load is 43°C. Pretty crazy, i thought my MBM was just wacky but apparently thats just the way it roles.