I think the stylists just help her pick pieces for her everyday wardrobe rather than completely style an outfit for every single day. She does look great every day but that's because her style is usually really simple and basic, like cardigans, jeans, t-shirts. She also sticks to a basic color palette. People tend to mess up their style when they start wearing clashing prints/textures, over-accessorizing, etc. and Emma doesn't do that.
Plus she always looks really well-groomed (hair, nails) and she has good posture so that all adds to the effect of looking very well put-together every day. There are other celebs who also wear jeans and t-shirts but they don't look nearly as good because they don't brush their hair or slouch. And she has a nice, proportional body with long legs so she doesn't have to worry about wearing things that will flatter her instead of emphasizing 'flaws' (I don't think they're really flaws, but some might -- things like broad shoulders or bow-legs or short/stumpy legs). It's not hard to look good when you have all that going for you (see the model street style thread... basic style/colors and great figure for clothes = almost always looking great).