I emailed Kia Canada and asked why there was a big tongue weight difference between the Sorento and the Santa Fe (which does 350 lbs TW and follows the 10% TW rule for a 3500 lbs trailer). They said that the companies operated separately and couldn't give provide answer.
My best guess is that when they were weight-rating the Santa Fe (sold as a two-row SUV) for towing, they didn't have to take into account the weight of two additional passengers behind the rear axle of the three-row Sorento.
This would be two 70 lbs children seated in the back row, who could still be seated comfortably back there:
Code:
350 lbs [Santa Fe tongue weight capacity] - 220 lbs [Sorento tongue weight capacity]
= 140 lbs / 2 passengers
= 70 lbs / passenger
Rather than uprating the rear suspension to take a higher tongue weight (and maybe compromise ride quality), they just decided to give it a tongue weight rating that doesn't correspond to the max towing capacity.
I would hazard a guess the Sorento could theoretically tow something with a 350 lb tongue weight reasonably well with passengers only in the front and middle, but wouldn't risk my personal vehicle to find out.