Yes, it is very odd. The indeed do serve burgers at night - and I've never had a wait person bat an eye at the request. One night I asked the server, and though she said she offically did not know, she shared my assumption that they didn't want to be seen as a "burger" place at night, and when burgers are on the menu that is what they predominately sell. It basicly has to do with the image of the restaurant (wanting to look more "upscale" at night) and I'm sure making more $$ on the other pricer dishes.
BTW, any time I have ordered a burger at night it has been the same price as lunch.
N.E.D.