Nike iD women's customisable shoes, size mix up when add to cart?

Hi so I just designed my Nike shoes, Nike dunk high iD, and I put in my size (uk) and then added to cart and it changed to 3 sizes down?
Is it changed from American or European sizes to British or vise versa?
Do I trust what it says in the cart or do I trust what size I picked?
Is there some way of working this out?
I've tried on an iPhone and Samsung tablet but iPhone goes up in size tablet goes down, I don't get it, what do I do? Had anyone else had this?

Call Customer Service for Nike.