A+B correspond to the city limits of Berlin, and BER is outside of the city limits. I'm not saying that this is set in stone. They could probably include BER i

A+B correspond to the city limits of Berlin, and BER is outside of the city limits. I'm not saying that this is set in stone. They could probably include BER in B if they wanted, but the most benign explanation is that it just started out this way because of how the fare zones were defined, and now they keep it that way because few companies hate extra revenue.