The AIM of the lights is typically different.
Is it possible that the angle that you see the brightness from, standing in from of the X, is affecting how bright they look, so that if aimed more at your eyes, it would look brighter than if aimed off from your eyes, etc.
Also, you can swap P/D side bulbs, and see if it changes with the bulb, or, if its the side itself.
If its the bulb, so its brighter with one bulb, no matter which side its in...just replace the dim one, it might be a wussy bulb.
If swapping bulbs doesn't change the brighter side, then, check to current at the socket for the bulb, to see if they are getting the same juice. Do this for the hi and lo.