I think it might have something to do with her physique. Her waist and everything is tiny. A lot of other women body builders are huge and look like men, she still has a womans physique.

Also when doing bench you are supposed to drive with your legs while pushing up, any time you push down on your feet, your back is doing to arch a little. Most guys bench pressing are so thick that you don't notice, like I said, she's like 3 inches thick since she's so tight.