my senitment on RDA values is that each person is different. the amount we absorb i believe varies, the amount each of us need as an individual varies. also the delivery of a specific supplement also has impact on it. i used to work for a lab that tested potency, and it varies from product to product. also, if you supplement for a specific purpose, the doages also change.
getting nutrition from food should be the primary way to get nutrients, but supplementing should do exactly that, supplement your diet.