Consider this; resin is only about 70% THC at best. So for weed to be 20% THC it would have to be about 30% resin. Hash itself is about that much resin, and it's almost pure trichomes. Obviously, it's impossible for bud and hash to be the same resin content, since bud contains a lot more than just trichomes.
I've made putty type black hash by extracting the resin from buds with acetone and mixing it with some finely powdered bud, as much as was possible to mix in and the hash still be kneadable. I consistently got a yield of right around 25% of the starting bud weight. BTW, to make black hash you need to extract the resin with acetone because butane or other non-polar solvents produce a resin which will crystallize, or polymerize or something, into a waxy substance which crumbles like fudge instead of being pliable. When you first make it, it will be pliable and normal but after a day or two of sitting it will transform into worthless wax-hash. Well, not worthless but inconvenient to handle because it falls apart.
So point being, if bud is only 25% black hash then if bud was 20% THC then black hash would be 80% THC, which obviously it is not. Black hash is usually 1/3rd resin or less. If that resin is 70% THC then the hash must be 1/3rd of that; about 23% THC. So bud must actually be 1/4 of that, about 6%.
There you have it, ladies and gentlemen. I have just proven mathematically that bud is really only 6, maybe 7% THC tops, to be generous. Oddly, that tallies with figures I read in a scientific journal from the 70s about Cannabis, which found that the most potent strains were Thai and certain Mexicans at 7%. That was the highest levels they found. Most were more like 3-5%. Now I ask you, could weed magically get 3x as potent since the 70s?