I only saw the fairywrens in the Semi-Arid Aviary, I didn't see them in the Finch Aviary. And of the two I saw, I only got good enough views of one to ID it as Purple-backed - the others could well be different. The signage calling them Variegated is technically not incorrect - the Purple-backed was long considered a subspecies of Variegated, but has been split relatively recently.
Admittedly, I am not terribly familiar with identifying species in this species complex, so I could well be wrong, but I certainly thought the bird I saw in the Semi-Arid Aviary looked like a Purple-backed.