On a synchronous motor the speed will always be the same regardless of the voltage, as long as the line frequency (Hz) is constant. Synchronous motors get their speed setting from the AC frequency, not from the voltage. This is one of the big advantages of synchronous motors, because it means their speed is unaffected by voltage variations. It is also why a European 50 Hz score motor runs faster when you use it in the USA, where we have 60 Hz.
If you run a 27 VAC 60 Hz motor at 16 VAC, 22 VAC, or 27 VAC (all at 60 Hz) the motor will always run the same speed. At the lower voltages the motor will have less power but the drive coil will run cooler. At the higher voltages the motor will have more power but the drive coil will run hotter.
So the change from 24 VAC to 27 VAC in the motor spec could be that Williams engineering decided that a slightly under-voltage motor would still have plenty of torque for the application and the extra head room for the drive coil would make the motor more reliable (especially, as others have noted, when the game was on high-tap).
Gottlieb did a similar thing, their score motors often being rated at 30 VAC even though the GTB transformers typically provided power several volts below that.