Is there a reason we must use a Volvo OEM 2" Hitch Ball -vs- after market quality Hitch Ball?

 

What I found out is the Volvo OEM Hitch Ball is Metric at 50mm which is 1.9685 inches. A 2 inch US Ball is 50.8mm. So the US Standard 2 inch ball and shank is slightly (.8mm which is less than 1mm) larger. A U-Haul 2" ball and shank works.