It looks to me as if the dress is actually too small and is undone at the back. Which would make sense as although Z is very fit and slim, she is likely to be v muscular as a sportswoman and has got norks, unlike sample sized moggels. It was prob only photographed from the front.
Which, admittedly, is hideous anyway. I think all those dresses are awful, and IMO she needs a below the knee number to flatter her legs.