It is more than likely the Mexican government bought the ammo which was then reallocated to the cartels by theft/graft.
The idea that gun products that can be freely purchased throughout America are ending up in Mexico, where you can literally walk across the border into Mexico through a turnstyle and they just have a sign that says if you have any of the following contraband please walk to our office located here instead of border officers actively reviewing each person walking through at official border crossings.
It’s not very shocking at all.