Amplifying what Pinball Wizard said...different games draw different amounts of current from the +5v supply. It's usually the heaviest draw, but also the largest variation, so the variation of the average value (as a percent of 5V) can be larger from game to game. The other voltages are set up as multiples of the +5v supply and the current draw is usually less which makes them easier to regulate. So they should stay close to their respective targets once the +5v is dialed in.