I have read similar things, basically the LD50 of THC is estimated because it can't be achieved.
Found a decent source.
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2100129/
Given that there is no known LD50 for cannabis, this should not be a major concern. Second, patients who choose to ingest cannabis will likely require significantly higher amounts. Third, until more refined and purified cannabinoid preparations are available, it will simply not be possible to derive a more specific or exact dosing schedule. Frankly, it was never our intent to advance an exact dosing schedule, but rather to provide some guidelines for the dosing of medicinal cannabis that were based on science and reason, not propaganda or agenda.
Edit: concentrates certainly change things a bit, but when the LD50 of the normal form is so high I have some legitimate concerns that concentrated forms would reach it. There might not be a realistic rating because, get this, it's safe. More research on concentrated forms is obviously needed.