Hide Forgot
This happends with an ATI Mach64 GB in a Dell Optiplex GX1. The default display mode Xconfigurator offers is 1280x1024@32 bits, but if you turn that down only the 8-, 16-, and 24-bit modes are listed in the next screen.
24bpp and 32bpp are internally treated the same by Xconfigurator. It will first try to determine a 32bpp mode, and if the driver doesn't support it, it will use a 24bpp mode. Therefore even if it describes the "24 bit modes", the mode actually selected will be 32bit if necessary. A bit confusing, and could probably use some work, but Xconfigurator is being rewritten anyway, so this problem will go away.
I got very confused about this too. I'd recommend changing "24" bit to "24/32" bit as a quick workaround; this would at least give a hint that 32 bpp hasn't just been forgotten.