Maybe the fact that pot, back in the day was the whole plant. When I bagged dope 35 years ago, it was the flower tops, and everything else. Being that the psychoactive stuff is concentrated in the bud, by including all the sun leaves it reduced it's power.
But I also know that growers have been cross pollinating and growing hybrids for years in the attempt to increase it's potency. All that work has not gone to waste.
Also, we never culled the male from the female plants to build those enormous buds.
There was no two hit reefer in the sixties, I would have remembered that.

Richard