Those ratings are conservative, to avoid any potential sickness or bad reactions.
Like most of us I've had plenty of kava from low-rated vendors. I used to love Noble Kava/Vanuatu Kava Store until they stopped selling kava Tanna and had stuff spiked with tudei. Even their medicinal marangmarang was good stuff.
The guys who get A ratings seem to be the ones who never compromise. KWK had some Tongan that was too low in KL% a few months ago. Not harmful, but not strong either. He never solid it publicly. @KrunkieMcKrunkface had some and said it was weak but fine.
GHK's ill-fated Vanuatu kava was from a new supplier. Small samples tested good, but when the big shipment came in, it was all spiked with Tudei. Chris took a loss on that stuff and did't try to pass off rubbish kava.
Those are some of the hallmarks of high quality vendors.
But as they say, "C's get degrees". I'm quite happy to sample kava from other vendors. Some other kava customers may not want to take that risk.
Ranking vendors and kava has always been controversial. Back in the day
@Deleted User used to have tons of test results here on KavaForums. It must have pissed some people off, 'cause those results are long gone. (Besides, they'd be outdated by now anyway.)