It actually depends on more than simple speed. For example, if the spurt occurred on a back thrust, those sperms will have a much greater distance to travel than sperm that are ejected on the most forward-thrust. So the most forward-thrusted sperm will have a serious advantage in getting to the egg first.
He could also have been one of those unlucky sperms that came out in not a spurt, but a dribble.
A sperm spurted from a back-thrust has a greater advantage than a sperm dribbled closer to the uterine wall, where the egg is chillin'. It's really all about distance, man.
Just thinking it through scientifically, of course.