To my knowledge (as someone from former West Germany), East Germany was ahead with incorporating women into the job market. Presumably, that is still the case. So, it could be that the friendships came to be from working together as colleagues...

That may just be the cause then? The difference in relative population? Since we don't know from the graphic what the numerical foundation is, it might be sometime that skews toward the "more common" side when there's just less women in an area.