Abomination,
You probably upgraded your mb(is it a kt7?) to a newer bios that includes compensation of some sort in an attempt to correct thermistor reading inaccuracies.
Bojo,
Temp reading software that works(unlike Sandra), read hardware information off a pc-health chip(like a winbond chip) or a motherboard's southbridge chip. These chips read cpu temp, or cpu thermistor temp, fan rpm, monitor the different voltages, etc. The software in turn gets its data from these chips.
Mike