Methinks someone incorrectly put in a two filament socket and bulb because he thought two wires mean two hot wires, when one was a ground (though an unnecessary ground). Maybe he wired it with two wires thinking you had to have a ground wire rather than use the trailer body as a ground. Then only one filament ever lit and the other never did.
When I read these threads about rewiring some part of an older trailer, sometimes it seems a previous owner did strange things and now the present owner has to make sense of it. Sense cannot be made of it, but it might be explainable if you can think like someone who knows nothing about wiring. I am semi-qualified in that regard.