I know why the manager was harsh - because they didn't want a man in a dress in the ladies dressing room. I pass very well and when I used Ross's ladies dressing room I sometimes went out in the mirror and I would pass ladies who were shirtless / just their bra showing / or sometimes a boob hanging out. Surely they don't want a man to see this.
So the best thing to do when trying out women's fitting rooms is to make sure you pass nearly 100%