Both could be relevant. It could be that a subgroup that makes up the majority of the military gets benefits, so the median is higher productivity. But due to a small subgroup, the mean is lower. Any result seems interesting here.
[ETA: Don’t you think something like, “People in the Army have lower productivity but people in the Air Force have higher” would be interesting? I just am looking for something that’s relevant to the central question of the post: can training have long-term benefits on self-discipline?]