I think you are right, THC levels can be affected by too many variables and in the same strain could vary pretty widely depending on environmental conditions. Also some of the recent research seems to suggest that THC alone is not as effective at treating pain as when you combine THC with other cannabinoids, this seems to suggest that THC content isn't the only thing to be looking at for 'best affects'.Edit: On second thought THC levels are overrated. Its all about the sample, the growing environment, the mood and the tools you use to get stoned. People tend to get off on grossly exaggerated THC levels when they should be concentrating on the reported user effects.