The computer isnt taking multiple fuel efficincy readings and then averaging them together.

The computer is measuring the total distance, and measuring the total gas used. So the distance goes up, but the fuel stays the same.

The number maxes out at 99 mpg, so even though you might be over that number for a while you will still show 99. However, eventually your average will pull down, and the "extra" milage you had when you were in EV is not lost, its averaged in, because again, its not instantanious measurements, its one big measurement