Hi there,
If MikeWarrior2 sees this thread, he'll give you the exact "thermistor-related reasoning explanation"...he's awesome. I'm not awesome, but I CAN tell you that Asus boards typically read higher than other boards. It's a well-known fact that others on this board will corroborate. Somewhere around 5C, I believe.
However, even 40C is way too hot for a case temperature. You have no airflow thru your case. That's your problem. Which case RU using?
What you need is a steady flow of air from the front, bottom of the case to the rear, top of the case. If you put two fans on the rear panel, under the PS, make sure they are both blowing out, not one in one out. That would get you essentially nothing. A much, much better solution is one fan on the front panel sucking air in and one fan on the rear panel blowing out. That will cool your case fairly well. Try it. Good luck.
EDIT
I just noticed that you said "using Sisoft to check temps." Do you mean SANDRA? SANDRA is notorious for bad temperature readings! Use either Motherboardmonitor 5 (MBM5) or the VIA Hardware monitoring utility that came with your mobo!