There is an article (I can post the link) that says it's a myth that pot has gotten more potent.
Those who are measuring it, are sent the MJ from random siezures. Mexican shwagg was more prevailent in the 70's & 80's. Most of the weed seized then was this schwag. With the appearence of the internet and more information being spread, there are more people growing sinse. Growing for themselves and profit. Now larger amounts of home grown sinse are being siezed, increasing the THC of the weed in the "tests" that they do. I can give the link if anyone is interested.