We all know that it takes time for a cold engine to warm up, and even longer for the transmission to get up to temperature. This is worst in the winter, but even in the summer can have a marked effect.
Because I have the ScanGauge fitted I get loads of instantaneous data, and what I have noticed recently is no matter how mpg conscious I am, I cannot get the numbers up, when driving on urban roads, until at least 4 miles have been completed. I find it a lot easier to get good "cold" mpg figures around town than on the open road. On the open road there is less scope for squeezing out a few extra mpg. If my regular run was 4 miles of urban roads in the morning, then 4 miles back again at the end of the day, I would be toiling to better 36 mpg average.
So the numbers you get are as much to do with the type of miles as the style of driving. The bulk of my driving at present is 36 miles of urban roads, then 36 back, with minimum cooling time in between. Hence my current excellent numbers.