The second best theory I have been exposed to for the adverse reactions to cannabis use in laboratory conditions dealt with the conditions themselves. The user's mindset and surroundings are extremely important when using any neuroreagent. Being in a situation where you are undergoing laboratory tests, perhaps along with the knowledge that you are officially being recorded doing something "wrong" (even though, of course, test subjects can't get in trouble for participating in an authorized test) could feasibly ruin the cannabis experience, and cause discomfort. Compare this to cannabis use in a pleasant social setting. I think introducing this as a variable control would show noticeable differences in the way in which THC effects people.
The best theory I have been exposed to, of course, is that those "results" are routinely fabricated to dissuade people from using cannabis, or heavily distorted by the media (as they routinely are with MDMA and LSD).
After reading the description of "Shwag", I thought it important to mention another rare but serious possibility as to the quality of pot:
It is not unknown for someone to combine "Angel Dust" (PCP and Formaldehyde) or straight PCP with Marijuana. Your chances of being SOLD weed like this without your knowledge are fairly small, since PCP is expensive. However, I have had the unfortunate experience of encountering someone who gave a girl friend (as opposed to "girlfriend") of mine a hit of the weed/Dust combo without her knowledge just to "fuck her up". I liken this to slipping Rohypnol ("Roofies") into a drink. This is an extremely reprehensible action, and not to be condoned for any reason. PCP is an extremely powerful disassociative (similar to DXM, Ketamine and Torbutrol, but much more potent), and its effects can be extremely disorienting and frightening if you don't know you have been exposed to it. Fortunately, I was there to calm her down and talk her through the experience (I won't even say what I did to the guy who gave her the hit).
The way to keep yourself and your friends safe from this sort of thing is to follow the same brand of common sense you would at a bar. Know the people you're partying with, have a reasonable idea of where your drug of choice (be it beer or otherwise) is coming from, and keep an eye on your glass (or pipe, etc.).