Ok found this on another site.. now I understand...
Light truck tires that are intended to be used in dual applications have two "Max Load" ratings branded on the tire's sidewall. This is because the load capacity rating of a tire serving duty in a "single" application is greater than the exact same tire being used in a "dual" application. For example, a LT235/85R16 Load Range E tire is rated to carry 3,042 pounds when inflated to 80 psi and used in a single application, but only 2,778 pounds when used in a dual tire application at the same inflation pressure.
This reduction in rated load capacity results in slightly larger tires being specified for the application that can better withstand the additional stresses experienced when a single tire goes flat and the three properly inflated remaining tires are required to temporarily carry the load at reduced speeds to remove the vehicle from immediate danger. This load reduction is not a concern for vehicles originally equipped with dual rear tires because the vehicle manufacturer factored it in when they specified the tire size for the vehicle.
2012 Cougar 330RBK
2004 Ford F-350