Her upper body is moving faster than her lower body. Hold her torso firmly against yours with your connecting arm and you'll notice a significant difference. She will, too. Just like you're adapting to her body, she's adapting to yours. Any habits will be reinforced by each other so that if anyone tries to improve his or her form/technique, it will be compromised by the other's faults. You'll wonder why it's not working and think it's your fault. But in reality, it takes two to tango. <--- This is a really apt aphorism in this context.