I've heard claims like this for decades, it was and probably still is a frequent claim made by cops and teachers giving DARE presentations in American public schools. It doesn't make much sense and never did, why would weed dealers be giving people more drugs than they actually paid for?
It sounds like a cope excuse people come up with on the spot when they don't want to admit what drugs they were actually taking. Ask an alcoholic after a car accident how many drinks they had: "I only had one beer, I don't know how this could have happened!" Ask a fent-head after the OD what happened: "I don't know, I only smoked a little weed!" Yeah right.
inb4 "dealers do it to get people addicted"; it's weed, it sells itself.