Nathalie has a point. Shops use very clever lighting and different mirrors to influence how you see yourself in shop changing rooms. So it is more than possible to try something on in a shop and it look great, only to find that in the "real" light at home, without a flattering mirror, it looks vile!
There are certain shops that get it totally wrong too... and have horrible light and mirrors, so anything you try on looks terrible, so you have to try it on at home to see that you LIKE it!!!
Its true!