I have a March and it does the same thing. I believe the aluminum body of the scope is the culprit. Aluminum has one of the greater expansion ratios of metals commonly used. Thus the scope is slightly longer when warm and shorter when cold. A math prof friend of mine stated that he calculated that my scope would expand/contract by 1.4mm with a 40 degree fahrenheit change.
I do not remember who's formula he used to get the numbers, but I trust his math.