This is a new feature of Displays 1.7.x which comes pre-installed on the Power Macintosh G3 1999 models. The Monitor Resolution control strip module now lists only Recommended resolutions by default.
Holding down the Control key while clicking on the Monitor Resolution control strip module will force it to list all available resolutions. If you choose a resolution that cannot be used with your display and the screen goes blank, press the Enter key on the keyboard or give the computer a few seconds; the display should change back to the previous setting.
For more information, see Mac OS Help located on your hard drive.