I agree with you ambi1.  I understand the angle the 6-pin takes in the ringing ten diagram.  But in a flat ten situation, the angle may be more "parallel" to the diagram. 

They way I understand the reason it is a flat ten is the ball deflects from the head pin more thus pushing the 3 pin more vertical than a ringing ten (in diagram 1).  The 3-pin then contacts the 6-pin more on the 5-pin side pushing the 6-pin in a more parallel direction.  (imagine cutting a ball more in pool)  Because of the reduced energy given to the 6-pin, it doesn't leave the gutter.