Don't know for sure the history but I expect something like since the allowable SM hours on most contracts are more than the actors, the salary is higher. We all know we put in hours and don't report them, perhaps this was a way to protect those who under-report...?
I know when negotiating LOA's to many agreements, we, the AEA committees, set the ratio at 20% higher for SM's and10% for the ASM.