or... how bout this,
its really hard to explain, so sorry if this doesn't make any sense...
when the ball hits you, the round front face (part that hits you first) is weak, (its a simple concept, domes are not a strong shape) resulting in little or no welt. but the sides of the ball are stronger, so they push on the skin, or maybe cut it a little, and that is why the welt is circular.
ok so the red is the face that will hit you (the pinkish tan), so the red is also the weakest spot on the ball, and the blue, is the strongest spot, so thats why it will make a ring.
let me know if you know what i am talking about