It is just the computer recalibrating distance to empty based on what mileage it notes you are actually getting vs. what it assumed when the car was new. No one is sure what average number it uses in this computation - mileage since new, mileage last tank, mileage over last 1000 miles? It appears it probably is some weighted average, weighted toward most recent.When I got my car (Early February) it used to read 362 miles as the distance to empty.
Now, whenever I have a full tank, it reads 340 miles until empty - a drop of 22 miles! Is this normal for a new car to drop like that?