Breaking the rules with thermal paste!!!

I have now tried this 3 times and the results have been the same. I clean the CPU/HSF thoroughly and apply the thermal paste to the heatsink instead of the CPU. Spread it thinly over the area occupied by the CPU and get a temperature 3C lower than applying to the CPU.
I am using a ThermoEngine and am baffled as to such a large difference in temperature. My only guess is that the paste makes a better and more even contact with the die.
Any ideas, and I would be interested to hear anyone else's results.

I dunno, if I had to guess maybe you are just spreading the paste better on the sink ie:more evenly, as it is probably easier than putting it on the cpu.