BIOS upgrade lowers CPU temperature

Setup

I've got a system with an Asus P8Z68-V PRO motherboard and an Intel Core i7-2600K CPU running at stock speed (no overlocking) which I cool with a Noctua NH-U12P. On the heatsink I've got the two included fans connected via the included Low-Noise Adapters (L.N.A.) 1100 RPM, 16.9 dB(A). In the BIOS settings I've set the CPU and chassis fan profile to silent.

Issue

Yesterday I upgraded from BIOS version 0501 to 0606. After the upgrade I checked the temperatures in the BIOS monitor and was surprised to see that the CPU temperature was slightly ~30°C. Before the upgrade the CPU temperature was ~50°C with the same BIOS settings (see the following heading for details on temperatures). How can this be? It seems a bit odd that a BIOS upgrade can lower the CPU temperature by 20°C and it also seems odd that the CPU temperature is lower than the chassis temperature.

Temperatures

When I've checked temperatures the room temperature has been ~23°C. I haven't changed the placement of the computer nor the hardware or cooling setup between BIOS versions.

BIOS version 0501

BIOS monitor:

• CPU: ~50°C
• Chassis: ~33°C

I haven't got any temperature measures from lm-sensors or the like for version 0501 because I only discovered the issue after upgrading to version 0606 and the BIOS updater utility won't let me downgrade to version 0501 (it says "outdated image" when I try to load version 0501).

BIOS version 0606

BIOS monitor:

• CPU: ~30°C
• Chassis: ~33°C

lm-sensors in Ubuntu 11.04 Desktop 64-bit (sudo sensors after an uptime of 4 h 52 min and a load average of 0.22, 0.18, 0.15):

coretemp-isa-0000
Adapter: ISA adapter
Core 0:      +32.0°C  (high = +80.0°C, crit = +98.0°C)  

coretemp-isa-0001
Adapter: ISA adapter
Core 1:      +35.0°C  (high = +80.0°C, crit = +98.0°C)  

coretemp-isa-0002
Adapter: ISA adapter
Core 2:      +29.0°C  (high = +80.0°C, crit = +98.0°C)  

coretemp-isa-0003
Adapter: ISA adapter
Core 3:      +36.0°C  (high = +80.0°C, crit = +98.0°C)

The BIOS monitor temperatures was checked directly after the lm-sensors temperatures was checked.

BIOS version 0706, 0801, 1101 and 3203

I get the same kind of temperatures both in the BIOS monitor and with lm-sensors in BIOS version 0706, 0801, 1101 and 3203 as in 0606.

Information from Asus

The 0606 changelog mentions nothing explicitly about CPU temperature (but item 3., as indicated by sidran32, might affect temperatures):

P8Z68-V PRO 0606 BIOS with IRST 10.6.0.1002

1. Enable the support of Intel Rapid Storage Technology version
   10.6.0.1002 Release
2. Improve DRAM compatibility
3. Improve System stability
4. Improve compatibility with some Raid card model
5. Increase IGD share memory size to 512MB

However the following FAQ might give a hint:

FAQs

I find that the CPU temperature reading in BIOS is about 10~20 degrees
centigrade hotter than the reading in OS. Is it normal? Page Tools

Solution

That is normal as BIOS does not send idle command to the CPU, making
most of the power saving features useless. You should be getting
similar reading if you disable EIST/C1E/CPU C3 Report/CPU C6 Report in
BIOS.