People in rural areas are not connoisseurs of art nowadays, and in earlier, much hungrier times were probably not interested in creating masterpieces anymore than rural farmers from Kansas are today.
Some people milled with a two-handed method using vertical strokes, others used big pestles and rotated. The two handed method would explain the "heart shape" (right). 3000 years ago Socrates wasn't even born, and he was the smartest guy in the world up until then... even he thought that the liver was where our intelligence resided. But these starving rural farmers probably had it all figured out and were trying to communicate it to us...
But I'm not really sure. Perhaps you should all send me money so I could study this further... a million or two should do...
When I'm done with this, I'll study crop-circles.