The 550E (which I assume you are running) only radiates 14.5 Watts. A heatsink with a C/W of 0.41 means that you would get an increase of 6 deg C over ambiant. A heatsink with a C/W of 0.345 means that you would get an increase of 5 deg C over ambiant. A heatsink with a C/W of 0.276 means that you would get an increase of 4 deg C over ambiant.
A total of 1~2 deg C difference de[ending on the heatsink.
Now possible answers to your case are:
1. That it is well within the measuring tolerances of the probe.
2. Fluctuations in the ambiant temperature could explain the differences.
Personally, I am not sure that an Alpha helps that much (if at all) with CPUs that radiate so few Watts. IMO, the Alpha is not worth the money in such cases.
In cases where the CPU is radiating a lot of heat (50+W), the Alpha is king.