Yup, Volvo knows how to do this stuff!
They're tapering the greenhouse above the wheels so they can reduce the rear wake area but still keep a wide rear track.
Honda took the same approach with the 1st gen Insight (max greenhouse taper above the rear wheels) but they
also reduced the rear track, like the GM EV1 electric car.
That bottom C30 you posted, though? That's not the good one. Wrong spoiler.
Better aero on the C30 "Drive" efficiency model: