There is a problem.
We don't know how those 20+% samples were prepared.
There is a financial advantage to claiming high thc numbers.
It's true that some growers are better than others, and as you decided to point out, I could be a terrible grower.
But when any grower establishes a baseline of potency over dozens of strains, and several years, questionable lab tests of a harvest particle are far less reliable than how we judge the potency using our own bodies.
When I grew Double Grape, my friends didn't like it.
They loved my other stuff.
I'd give it about 12% thc, but there is thc variation among phenotypes, and the average of several grows would likely be higher.
Growers Choice claims 26% for their CriticalPurple auto.
I didn't believe that number when I bought the seeds, and my suspicion was born out when I grew it.
it's really in the typical 15-20% potency like most other high thc strains.
THC numbers are highly suspect.