There are a few things. For one, Jennifer clearly doesn't care much about fashion; Leelee looks like she does care about fashion at least a bit. Jennifer probably just puts on whatever her (very laz) stylist tells her to as she's just an ambassador for the brand; Leelee wears Christian Dior clothing because she has followed Raf Simons since he was at Jil Sander. I don't know if Leelee has a stylist but she makes it look like she doesn't. Maybe she just has an innate sense of style, though if you look at her style from a few years ago she looks like a completely different person, which is interesting.