Maybe the new gladiator has a much better luck score (secret stat). Also you have to get the opponent's healt to 0 to get a chance to kill them. So if the opponents give up on 1 health you don't get to kill them. As Lunarion said, fighting strategy and Presence matters (mostly booodlust in strategy I guess). There are a lot of factors that determine the kill outcome. Maybe a lot of time the opponent gave up before the 0 hp condition was met.