Look on page E6 in the manual... one of the last pages ... sorry some text isn't pasting right from my iPad:

Your game’s CPU board uses a 3V coin cell, lithium ba ery (CR2032) to maintain its basic input/out- put system (BIOS) se ngs when the game is powered down. If these se ngs are lost, the CPU will not boot when the game is powered up. The life expectancy of the CR2032 ba ery is approximate- ly three years. It is important to change your game’s CPU ba ery before it discharges below 3V. However, in order to preserve the CPU’s BIOS se ngs, the change must be made while the game is powered on. A step-by-step process for replacing the ba ery is provided below.

One other difference that may be there (speculating) ... with the switch off on the pinball machine, I don't think any power is being supplied to the computer. With standard desktops and laptops, even if off there is a small amount of power to the motherboard which reduces the discharge of the battery.