The GP didn't exactly explain it well, but they (I assume) are referring to the manufacturing process. There's a lot of reported issues about QA controls. If you buy a bottle of asprin, it's pretty safe to say that each pill will have very close to the same amount of active ingredient as every other pill. The tolerances are pretty low because of tight regulation.
Herbals don't have the same level of regulation, if any at all. You have to take on faith that the manufacturer is being truthful that any given pill contains the exact amount, or even of the exact herb, they say it does. If you google "fake herbal medicines" you get report after report of various places selling false herbal medicines.
Even with a legit manufacturer, it's still up to their own internal processes to get things right because there is almost no external oversight.