In reply to Postmanpat:
I don't know the answer, but here is some guesswork to the logic.
If it was my girls, it would helpful to them so they can buy a dress 2 sizes too small, so they can diet to get into it by the summer.
When/if that fails they can take it back and get the rights size.
(If they only drop 1 size, then they can take it back and get one size bigger)
If they actually do get down two dress sizes by summer then of course they will now fit it, and the incentive has worked, no need to return it.