I'm seeing the same thing. I wonder if it's related to the FAQ in the README:
Q. Why can I not get a resolution of 1600x1200 on the second display device when using a GeForce2 MX?
A. Because the second display device on the GeForce2 MX was designed to be a digital flat panel, the Pixel Clock for the second display device is only 150 MHz. This effectively limits the resolution on the second display device to somewhere around 1280x1024 (for a description of how Pixel Clock frequencies limit the programmable modes, see the XFree86 Video Timings HOWTO). This constraint is not present on GeForce4 or GeForce FX chips -- the maximum pixel clock is the same i on both heads.
It looks to me like it might be mis-applying this test on my Geforce 6600GT.