It might be the adult colouration? Although he's actually looked like that at the store when he was maybe 3". So if it is a juvy/adult thing then this one turned adult colours pretty early. Mind you I have seen orange shoulder tangs at 3" and you'd think they'd still be the juvenile yellow at that size so I guess adult colours can come pretty much whenever they feel like it in the fish world.
A couple things that I have observed however between seeing lavenders in the stores, other peoples tanks, and in Hawaii - there do seem to be two major colour morphs. A darker one like this one, and a lighter one where it's more of a pinkish grey in the body with a purple/pink/lavender border along the fins and so on. (I'll see if I can dig up a photo.)
But, lighting can play tricks on you. I've had people see this fish in person exclaim that they didn't even realize the fish was that colourful because between swimming so fast and it being hard to look at because he won't sit still, and the light needing to shine on him just right, he can appear like a brown colourless tang unless you're really looking at him closely.
Of course there's also that fish store tanks are stressful places for fish with seldom the most optimal lighting conditions to showcase subtle colours on fish, so that could be a factor too maybe..