360 and 1080i/p - not filling the whole screen!?

I have my 360 hooked up to 1920x1200 LCD screen so I expected to have a little black bar in the top and bottom but it's much more than that!

It seems the 1080 signal, regardless if I select 1080i or 1080p, may be in telling the screen it's in full 1920x1080 resolution but part of the signal is just black. Regardless if I'm gaming, in the Live GUI or even playing one of the small Arcade games it's like the 3600 is giving me something like 1600*900 with a black frame.

Any one else have this?

Kind regards