Lots of possible explanations. My mind initially went to small sample size, etc, but it looks like even assuming a 50% sex ratio (which, isn't quite right for bison, there are slightly fewer males than females) the probability of this result is around 0.26%, which is possible since there are a lot of fossil researchers out there and this is the type of result that would get press over a result that came to a more expected conclusion, but still. It's less likely than I initially would have expected.

If I had to guess I would have to assume it's something in the nature of how bison die. Say from male-on-male competition far from predators that may break the bones to digest the bone marrow. Or it's possible that predators prefer preying on females because it's relatively more safe than males.

Or it could just be the small sample size, of course.