Some grey water to black tank....
Reasoning is for black tank to have high liquid to stuff ratio.
That was 'normally' done with a bathroom sink drain.
Also came about due to 'plumbing drain line' issues......floorplan obstacles so sink drains were easier to direct to black tank vs grey tank.
Sound like your floorplan shower drain is easier to plumb to black then plumb it to a grey tank.
Grey tank location, framing, floor supports, ducting etc play a part in plumbing line installation so proper pitch can be achieved.
Shower draining to black tank....NOT ideal but doubt you will/can change it due to items ID'd above.
A quick 'visual' inspection of where shower drain is, where grey tank is, where black tank is and then how shower drain line is plumbed will give you a better understanding.