Perhaps you're confusing THC with synthetic cannabinoids. The molecule doesn't care whether it's made by plants or at a lab.
Plants yield a mixture of cannabinoids at some concentration, which constitute ~10% of the dried plant matter. These can be extracted and separated with various techniques. The effects of pure THC are different than that of the mixture.
Upcoming schemes use engineered micro-organisms to produce specific cannabinoids, efficiency of the systems is ramping up. My understanding is that the synthesis of cannabinoids is is quite an inefficient way of manufacture.
None the less, it's all the same - covalently bound atoms :)