The use of two different terms to describe the same condition...
You're assuming that the two phrases are in fact describing the
same condition. I think that's only the case if you're taking a very short view that either one only means you can't get what you want.
I chose to assume that two different phrases are used because there are two
different conditions-
Currently Unavailable meaning they may be available again, and
Sold Out meaning they won't be available again.
My belief is that Apple will offer
some colors in
some bands continuously (such as the black Sport band) and other colors in other bands will be offered in a deliberately limited, one time basis. And/or some colors may disappear from one type of band and show up on another when one of that band's offering disappears. All that may be wrong. If not, it might serve to give some cach?t to Apple bands.
Some colors might be seasonal, similarly to clothing fashion, and return in a year. Maybe.
Maybe Rene knows someone on the inside who has actual knowledge.