Decants, never, have the same smell as that of the original fragrance.
That's worded quite strictly. In theory, you are right:
a) Oxygen changes a scent over time. However, that's true for any opened flacon, as each pump gets air into the body. Especially if decanted with a syringe, there is not much of a difference.
b) Smaller volumes of perfume change faster than bigger ones. However, a decant of e.g. 10ml won't be much different from bigger flacons over significant amount of time, especially if stored appropriately (rather dark glass than those white sprayer which also aren't completely tight) .
I have decants for years and cannot detect any difference in scent. Of course, it might be my nose
But I'm convinced enough to blind-test that with someone else.
As for the naming sample vs decants, we had the same discussion in the German forum. It turned out that there are two opinions, which we couldn't really unify.
1) Sample = Original, decant = coming from the flacon (could be from a personal one or as a service in a perfume store)
2) Sample = small size, decant = same as above, but usually 5ml+
I personally prefer version 2, as I find that both in German and English language, it's more intuitive (sample = I want to try, no matter if it comes from my friend or the vendor).