Preferred resolution for CRT1 output

[BertSmulders]

I have an AMD Radeon 6310 card with the fglrx driver. If I connect any monitor, to the CRT1 output and run 'xrandr -q', the reported preferred resolution is always 1600x1200, regardless wether the native resolution of the monitor is 1280x1024 or 1920x1080. Why is this and can it be changed?

Thanks!!