That claim isn't a known fact, and I've heard many different numbers for it. Even if it is, the difference tends to be that the amount of tobacco smoked (average smokers smokes like a pack a day, or 15.x grams) is far less than the average pot smoker (I'd consider over 3.5 grams a day very heavy use, while the average is probably 1 gram).
The problem is, I doubt you will find many people that smoke pot who do not smoke tobacco.
My friends might be an anomaly, but more of my pot smoking friends don't smoke tobacco than do. Like me, for instance.
Also the THC remains in your fat cells and afflicts your brain function (Although it has not been determined exactly how) for several weeks even after you stop smoking it. I know when I smoked often, even when I was sober I would feel to be in somewhat of a dreamstate. I could function well, such as with driving and such, but my thinking was somewhat off.
The claim that it has after effects for several weeks is very debatable, and is only supported by personal ancedotes, which do not apply to everyone.
(*source: used cigarette butts)
Not the claim that the THC remains in your fat cells for that long.