Question on Yael & Sisra

According to the rabbinic observation (based on the number of words Devora uses when relating nafal beyn ragleyha) there were 7 rounds between Yael on Sisra. Has anyone ever found a reason why this point is identified. I know she was tiring him out, but the number seven usually is signifiant and would seem rather much under the circumstances. And BTW this particular drush was not at all censored in my BY education. In fact, one girl questioned why Yael was concerned with avoiding kley gever in using a tent peg rather than a sword to slay the general when she already gave herself over to him 7 times.