Comment by kijin

I parsed it as ((mass grave containing 9 skeletons) dated to the early 1300s) && ((5 skeletons) dated to the 1300s).

As in, the grave itself was dated, and 5 of the 9 skeletons found inside were also individually dated to roughly the same century. Maybe the other 4 were not tested, or turned out to have been buried at a later date.