Well, ya gotta remember, the actual booting phase is one of the heaviest phases a computer goes through when in use. At the bios, it hasn't started all the hardcore loading of an OS. So of course the load is very light on your computer, thus the temps should be lower. After booting an OS, having programs running in the background constantly, of course the load will be heavier, thus higher cpu temps. So I'm sure speedfan's accurate since it's probably using the same heat sensors that your bios is using.