ok so mrtskiers response, i think is partly correct. i am not positive, but i am fairly sure that that the thc content actually may be the same, if not increasing by a little bit.
the thing is, somehow when it is smoked now, somehow (i tihkn its in the growing) more thc is being released then it was back in the 60's and 70's. so when combusted the weed back then would release i think, like 10-15% of thc. these days, when smoked it releases 15-20% and sometimes around 25%.
so in a sense it is more potent when smoked, and there have been new strains developed that do indeed have more thc.