It is not uncommon.
One benefit is that it avoids snaking the pipe over the rear axle -- though this is not a problem for the factory.
I suspect it is actually for the purpose of dumping the exhaust into a good air flow, to carry the fumes away from the interior. I ran Millicent's pipe over the axle and out the back, and we have been breathing exhaust fumes ever since because of the unfavorable airflows and pressure zones behind and inside the bus.
Drawback is that it takes up space where you may need to put a waste tank or something else.
I put Millicent's sewer tank in that spot, so I fabricated a "snake" over the axle and out the back. Much cutting and welding. The job included reorienting a brake chamber (not amateur work), plus correspondingly longer air hoses.