Parting Line Height, some pages back in this thread, you will see profile pix of Shock and Volt and you will see that Shock's PLH (i.e. edge of nose) is bit higher than Volt. In MVP driver's case, higher PLH tends to be more stable. You can put discs on i.e. table and see differences in PLH.
Do you have OAT issues? Some OAT in throw might turn disc over than expected.