why some airplanes have their exhaust gases vectored above the wing?
Assuming that both props are rotating clockwise as seen from the rear, the orientation of the exhaust pipes makes sense if the goal is to ensure that the exhaust gasses are kept clear of the wing.
The rotating propwash will tend to carry the exhaust from the right-side pipe (in rear view) of each engine well below the wing.  If the left-side exhaust pipes were similarly positioned, their exhaust would be carried up by the propwash to impinge directly on the wing.  The position we see in the photo helps ensure that the propwash carries the exhaust from the left-side exhaust pipe of each engine well above the wing.
A comment from another ASE member (John K) has stated that exhaust impinging directly on the wing can lead to galvanic corrosion due to carbon deposits.