Bottom line, the longer the arm the less distortion than that which would be created by a shorter arms geometry. The 12" arm would track in a shallower arc allowing the cartridge to trace more accurately from the beginning to the end of the record surface.
Think in terms of a 9" arm representing a circle 18" in diameter and the 12" arm would create a circle of 24" in diameter. Cut and paste (in your minds eye) a section of each on a record and you will see the difference each arc makes.
Think in terms of a 9" arm representing a circle 18" in diameter and the 12" arm would create a circle of 24" in diameter. Cut and paste (in your minds eye) a section of each on a record and you will see the difference each arc makes.