From the study:

> Twelve of the 14 drug compounds tested (86%) were present in concentrations at least 90% of the labeled amounts, the generally recognized minimum acceptable potency. Three of these compounds were present at greater than 110% of the labeled content. Two compounds (aspirin and amphetamine) were present in amounts of less than 90% of labeled content. One compound (phenacetin) was present at greater than 90% of labeled amounts from 1 medication tested, but less than 90% in another medication that contained that drug.

http://jamanetwork.com/journals/jamainternalmedicine/fullart...

Sounds like maybe concentrations are allowed to vary a bit from the label at manufacturing time? Not sure, maybe someone who knows more can comment.