Those aren't mutually exclusive, so the answer to your question is both. Most test results that show results ranging from like 10-25% (applying to the OPs question) refer to THC% of the dry weight. If you compare that % (of weight) to the total % of cannabinoids (included in the better test results) you know the % of THC of the total cannabinoids (which is typically specifically mentioned if that is the case). Those results typically show roughly 55-95% refer to the % of total cannabinoids but can vary much more especially in high cbd strains.
Maybe my first link now makes more sense, combine it with post #8 which pretty much nailed it already:
http://analytical360.com/testresults
At TestLabAmsterdam they take exactly 0.1 gram of mj and 0.05gram if it's hash and the % in the test results (typically ranging from 13-20 for THC) apply to that weighted sample.
Both total cannabinoids and individual cannabinoids are expressed in % of weight. Proof is in the math of thousands of test results (add up the individual % and you'll get the total % (of weight of part of the sample you provide). If THC% were a % of that % you'd still have no idea of how much THC was in the sample you gave.
Bunch of stoners!