Is it normal to have such discrepancies between bios and software readings?
Yes - but which is correct?
Personally would believe core temp.
If my memory hasn't totally failed me - remember reading an article some time ago that explained in some technical detail why you should not believe motherboard temp readings.
Have memories of watching alleged temps go up ,down ,up and down after bios updates.
Am running dual core Opteron and the difference between the two cores is approx 6c which , having read up , I understand is quite normal - something to do with uneven heatspreader.Would guess yours is o'k