I notice similar effects in my 05HCH. My theory is that as the tank passes the 1/2 way point, the computer accumulates more data points. More data points = more accurate results. If you don't reset the trip/MPG odo each time you fill up, you will notice a more stable MPG reading with time. When I first fill up my tank and reset the trip odo, I notice mpg reading anywhere from 50-70mpg.
If you have an A/B trip odo/MPG meter, try keeping one of them on long term without resetting it each time and the MPG data will eventually smooth out.