Leaf and substandard grows are now sold in bulk, cheap, to people who process it and extract the THC that is present, getting the final product in the mid to high 90’s percent purity, and sell it.
Back in the 80’s, I once plucked a bunch of leaves off a lone plant growing all summer on a hill in Meigs county, OH. Took that leaf home and dried it out quickly in an oven, and that stuff really put me on my butt! Tunnel vision as well. Just from the leaves!!
You just never know. :)
The genetics of the plant is a factor. In the fine tradition of the American can-do spirit, even hippies were experimenting with selection for potency. Europe wasn’t as hampered by illegalites. Weed has been selectively bred for a lot of different characteristics...yield, variations in photoperiodicity to match plants to the latitude they’re grown at, appearance, characteristics that make them suitable tor indoor growing. Horticulturists love that stuff.