Depends on who's doing the labeling. There are some opening positions that can arise from openings as diverse as the Caro-Kann Defense Panov Variation and the Nimzo-Indian Defense. A few openings like the Bogo-Indian Defense have almost no theoretical significance (ie very few "genuine" BID lines) and often transpose into some other opening.
In general the ECO classification is very accurate since it's a systematic code and not just a jumble of names that players don't always agree on (ie The Traxler Variation in Europe is often called the Wilkes-Barre Variation here in the good old USA)
BUT sometimes it's hard to find just what you want in ECO if you don't know the exact move order ECO uses to "define" an opening.
Offhand I'd say the final iteration of an opening is what counts, not what it started out as. For instance:
1.f4 Birds Opening 1...e5 From's Gambit 2.f4 Kings Gambit (by transposition) 2...d5 Falkbeer Counter-Gambit or 2...Bc5 Kings Gambit Declined or 2...ef4 Kings Gambit Accepted.
1.e4 e5 2.Nf3 Nf6 Petrov's Defense 3.Nc3 Nc6 is now a 4 Kts Game. BUT after 4.Bc4 Bc5 5.d3 It might be considered an Italian Game (Guico Piano). If 4.Bb5 it's a 4 Kts Game.
And what's in a name? Is 1.d4 Nf6 2.c4 e6 3.Nc3 Bb4 = Nimzo-Indian Defense but after 4.Nf3 d5 it's a QGD Ragozin Defense.
Are you confused? Wait till you try figuring out the differences in opening move orders and transpositions (and what they end up being called) between BCO, ECO, MCO, NCO and NIC.
Are we having FUN yet??
If a game begins in one form: lets say C42 Petrov's Defense, and transposes into another form (C57 Two Knights/Classical), what is the official opening labeled on the game?