Temperature info in Overdrive tab with catalyst 4.2 wrong? (9800xt)

Hello!

I got a 9800xt a few days ago (a "real" ATi one). When installing the
card I just removed my previous card (Geforce 4200), booted, installed
Catalyst 4.2 drivers (from the ATi webseite) and rebooted again.

Now I wonder if the termperature info in the Overdive tab in advanced
display properties is correct. It always displays 20° more than
RivaTuner does and to be honest the temperature info from RivaTuner
sounds more realistic than the ATi one from the Overdrive tab.
According to Overdrive tab my card runs at around 70° and it seems to
overclock the card until about 74° or 75°. I also installed the Zalman
heatpipe ZM80C-HP instead of the retail cooler, at first without the
additional OP1 fan and the card would go to 90° (!!!) accordiing to
Overdrive tab - and still work... now I got the OP1 and it's down to
69° in Overdrive tab and 49° according to RivaTuner.

Is this a problem of Catalyst 4.2 or maybe somehow related to me not
uninstalling the NVidia drivers first? (I haven't tried other Catalyst
versions yet)