It can be one of many things.
1) She's so in fashion. She's so stylish. He's so out of fashion, he's not stylish at all.
2) She's so ready to be in a relationship with him. He obviously doesn't like her.
3) She knows something he doesn't. she's in on the secret, he's not.
4) She knows what she's talking about/she maybe knows what he's like/feeling, he hasn't a clue what she's like/feeling.