I don't know why they like to style like that so much, but SOMETIMES, that slightly hunched posture is supposed to project a 'come hither' look that's slightly softer than the 'come hither' look in rancorous hollywood of the 50s onwards ala Mae West and the entire bevy of on (and off) screen hollywood sex symbols.
The main problem is that the shoulders and entire chest gets truncated, soooooo, only more nslender models with well-developed boobs should attempt that posture.