Here's my guess: A dimmer is an electronic device that has a voltage drop of maybe 2 volts. On a 12 volt circuit, the max voltage left for the bulbs is 10 volts, or about 90 percent. The same 2 volt drop on a 120 volt circuit leaves 118 volts for the bulbs, or about 98 percent. On a 12 volt circuit the lights would be dimmer.
