The dash display is more a rough guide than an actual result, if you want accurate readings you need the number of miles, the number of fuels and a calculator.
However, given the time of year, although the MPG may appear higher, the cold will affect the efficiency, but as well as that, the road conditions affect the resistance.
If the MPG is as high as it looks, then the engine is driving efficiently, however, if the roads you are driving on have increased resistance, all that benefit is lost as your moving the car, so then you get more MPG and less miles.
Check your tyres, tracking, air pressure as a start point, make sure they are at an optimum and then monitor from there.Also do the MPG calculations manually and compare to what you see on the dash. The Dash can read differently also as the dense cold air can fool the car into thinking its using less air, so it injects less fuel to compensate, thus the car sees its going further using less juice when in fact it isnt.