I was in Debenhams the other day, looking for a frock for a wedding. Sometimes a 10 fits me, sometimes a 12. I picked up three dresses, in both sizes. When I tried them on, one of them wouldn't zip up in the 10, struggled in the 12 (looked like an over-stuffed Christmas decoration), whereas the other 2 styles fitted oaky in the 10, better in the 12. All the dress were the same sort of style (strapless prom dress).
It's bad enough form shop to shop, but in the same store in the same range, what chance do you have.