How long is your average journey?
2500 a year is incredibly low, so is it possible your journeys are very short too.
The average MPG is always based on that singular word.. 'average'.
If you are not getting up to decent mileage and speed, then it will always be based on the most recent trips.
My Jazz is just over 2 years old, has done 27k and the shortest journey is around a 6 mile round trip, the longest (recently) a 70 mile round trip. I have been seeing 66mpg over winter, 74mpg last summer...
but. it will take a few miles of driving (even in town) for the mpg to climb to around the 55mpg mark. In some instances early on I only see 30mpg.. but thats because the average hasn't had time to settle.