Ø is used as a symbol for average value: average in German is Durchschnitt, directly translated as cut-through. (cut through zero)
MPG with the 1 in the corner is average MPG for your journey, and is reset when you manually reset or when the engine is turned off for more than 2 hours (i think) - so if you stop at a service station for example, the journey average continues to be calculated when u start the engine again.
MPG with the 2 in the corner is average since you last reset it as said above, and i also do this when i fill up, the 2 then becomes average MPG for the tank.
then obviously the MPG with no number or symbols is the live as you drive figure changing every couple of seconds.