Are natural cannabis and medical cannabis different?

Not really: Cannabis is a plant, which is cultivated in many forms. Its potency varies with the THC content of the particular strain of marijuana plant, and how it is ingested. Medical marijuana is obtained via a doctor's prescription and is legally obtained at licensed dispensaries in states where medical use has been deemed legal. It is not chemically different from "natural" marijuana.

Yes: What you get off the street is not pure or pharmaceutical grade. Perhaps home grown is close to the purity standards of a lab, but the results may vary from crop to crop as the growing methods may change.