I understand your question, but think you're missing the point. It's similar to, "What is the difference between an individual disassociating themselves and being disfellowshipped?"
They are technically different, but for all intents and purposes they are the same in regards to how the DF'd/DA'd person is treated.
From the Insight book:
APOSTASY: This term in Greek ( a·po·sta·si′a ) comes from the verb a·phi′ste·mi, literally meaning “stand away from.” The noun has the sense of “desertion, abandonment or rebellion.”
Technically, a person cannot be an apostate of a religion if they were never a member; you can't "desert, abandon or rebel" against something you were never part of. But JWs will shun anyone and everyone that disagrees with the GB's "Truth du jour" whatever that may be, so there is really no practical difference.
Keep in mind that one of the hallmarks of a cult is the redefinition of words. The WTBTS has a long history of doing this whenever it suits there purposes.