I still think that there is a problem with your horizonal & Vertical sync values. Here is the setting for my Philips Brilliance 109P monitor. It too handles 1920x1200 maximum resolution.
ModelName "Philips Brilliance 109"
HorizSync 30.0 - 95.0
VertRefresh 50.0 - 160.0
So it seems that there is a problem there somewhere. I guess that Linux will try to calculate if those values will support the resolution you specified, and if it's wrong, it will lower it to a value it will support. As you know, the monitor plays a major role in what the maximum resolution you can see on your screen. Even if the video card will support higher than the monitor will.
I haven't bothered to try the 1920x1200 mode yet, as the 1600x1200 mode the font is so tiny, I hate to see what it looks like at that resolution.
Check those values again. It may also be possible that Sun has putup the wrong values for that monitor. As I don't know the exact model, I can't confirm that.