I have worked in a bridal salon as a seamstress and have also independently altered dozens of wedding gowns over my sewing career. It is rare when a bride tries on a dress and it fits her perfectly, but I have seen it happen.
Wedding gowns ordered off the rack are not made to order, the size that will fit you is determined and ordered. From there, the dress will typically need alterations. Bridal alterations at a salon are very expensive, so if one knows a qualified seamstress and wants to save money, get the dress altered elsewhere.
I hate to admit it but there was one bridal salesgirl that always ordered one size up to promote extra fees for alterations and that angered me to no end. As a matter of fact that was one of the reasons why I left that bridal salon. This is not typical, but be sure you discuss size with the bridal salon before ordering.