I think something that alot of people here are daily to mention is that the reason you don’t see alot of % is that it all depends on the grow.
When you see an advertisement for 20% thc that was just the highest they found in a sample grown.
Here in Canada we have alot of testing going on and even at our local hydro stores they ussualy have a consultant that comes in from time to time and takes samples to get tested at a lab.
You can see that as you become a better grower you can dramatically change the THC and other cabanoids within the exact same plant/genetics. Let alone the fact people are growing from seed, some growing in soil, some in hydro, some are doing little tricks while feeding top end nutrients and some are just letting it buck in their back yard or in a big pot of soil like fox farms.
The other thing is that terpenes play a huge roll, so you may have a 20% thc bud that is lacking terps vs a 16% bud bursting with terps and the stone will be even stronger.
It’s not so cut and dry man, it’s like a fine wine.
I believe @PurpNGold74 has been learning alot more about terps maybe he can share some knowledge