Most of the OEM hardware monitor programs won't show the thermal-diode temperature for the simple reason that it IS higher (and frightens people). Then there's boards like the Abit NF-7 that use a dedicated over-temp protection chip that doesn't even report the temperature. And let's not forget that manufacturers like to 'adjust' the data (Asus is famous for 10 C changes with every BIOS revision).
I suggest a rough calibration of your thermal-diode. Notwithstanding the recent fun of C/W values varying with heatload and temperature, you can make a plot of multiplier vs. delta-T (diode - water). At a theoretical zero-multiplier, the delta-T should also be zero. If it's not, it gives a rough estimate of the thermal-diode's error. See also
pHaestus's first thermal-diode article.