The USDA test uses a chemical definition of Vitamin C that is very narrow and specific.
On the other hand, your body uses a number of related chemical compounds as though they were Vitamin C. In chemical terms, these are called “analogs.” The type of Vitamin C that’s in Goji berries is an analog of Vitamin C that doesn’t fall within the strict USDA definition of Vitamin C. But your body still recognizes it as Vitamin C and uses it as though it were Vitamin C, and it has the same effect as Vitamin C.
I never read or heard it explained in this way but it makes sense to me. You see you can make synthetic ascorbic acid by taking four hydrogen atoms away from refined white sugar. Ascorbic acid can be refined, refined white sugar. A lot of people including nutritionists seem to buy into it but it doesn't seem healthy to me. There is a big company somewhere that makes a lot of ascorbic acid this way and sells it to food and drink manufacturers.
I think the public has been duped by the food industry.