Motor Trend doesn't know. From slide #4 of 15. "For unknown reasons..."
"In 2010 Toyota did another redesign. Sadly this meant the death of the 4.7-liter V-8. Continuing on was the 4.0L as the only powertrain option. That could be because of the fuel economy standards. Despite the tow rating, the 4.7-liter 4Runners get fairly abysmal fuel economy. The tow rating for the fifth-generation 4Runner was set at 5,000 pounds. For unknown reasons, in 2014, the tow rating for the fifth-generation 4Runner dropped down to 4,700 pounds. Even stranger, in 2016 the SR5, SR5 Premium, and Limited 2WD's tow ratings jumped back up to 5,000 pounds; 4,900 pounds for the Trail, Trail Premium, and TRD; and 4,700 pounds for the Limited 4WD. And in 2017 the tow rating for all 4Runners settled at 5,000 pounds. Curb weight for fifth-gen 4Runners ranges from 4,400 to 4,805 pounds."