Iirc, he was an extremely small amount that would so its possible she just missed it. Also, she benefited from her employer being real powerful that gave her more leniency for some rules that she may not have known existed. If you don't know Russian, then it would be damn near impossible to know the regulations by the letter of the word. I would imagine that her having THC was previously allowed for flights in Russian because of her employer said so.

Edit: Some of the post is a bit speculative, but I don't think its farfetched and gives some benefit of the doubt when people are hellbent on running her over with a bus.