Bones

Great question.  For most of us righties, the spot of the 1,3,5,6,9 are critical.  Since four of those (with the exception of the 6) are the only pins that a strike ball contacts, the spot of them directly influences how they make contact with the rest of the pins.  While most bowlers can determine if a pin is spotted slightly left or right, few can determine if a pin is offset forward or backward.  The 5 pin spot is often the cause of solid 8 or 9 pins on a ball in the pocket, and the 3 or 6 can leave several versions of 10 pin.  A variety of other miss-spots can leave 7s, 4/9, etc.  When you add in all of the other affects of such variables as ball speed, axis tilt, rollout, deflection, carrdown, etc - it's no wonder that we sometimes think we've thrown a great game yet still can't figure out what happened to our carry.