I would be willing to bet that even the rounds that do fire follow different trajectories and their ability to hit something is not what it should be.
Possible problems:
Wrong powder - someone may have elected to use what was on hand rather than the powder specified. Bad shooting.
Inaccurate powder measure - Probably the problem. Bullets going all over the place.
Could be some old surplus powder that they thought might work.
Bad primers.
Inaccurate bullet seating.
These may be handloaded rounds and they could be highly variable.
or all of the above....