There's voltage drop in the wires going to the lamps.
I notice this on highbeam in my Fairlady because hitting the "Flash to Pass" button on the stalk lights up the brights better than just having the brights on alone. Two paths for current to pass through, more power to the lights.
Try setting up an H4-Style Relay system, you would be amazed at how much brighter even simple halogen sealed beams are when you have real wiring and a 10 gauge wire right off the alternator delivering the power directly to the lights!
Either that, or you still have sealed-beams...
In that case, I'm sorry!