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It's more subtle than that, though. Being above the threshold is not forbidden per se, instead the athlete has an opportunity to explain what happened, and then, if the UCI anti doping tribunal aren't satisfied, they can open an anti-doping violation. They can also clear the rider too, if they are satisfied with the explanation.
With a banned substance, that element of doubt isn't there, a ban is issued, irrespective of the athlete's explanation.
One is a banned substance, where even a nanogram is considered a positive, the other is a controlled substance which means it can be used up to a threshold level. That's a major difference.