One thing that has bugged me about most 17"-19" inch LCD monitors have 1280x1024 pixels as their native resolution - this isn’t the standard 4:3 ratio you see in TV’s and other monitors - I know most 15" LCD’s are 1024x768, and the 20" are 1600x1200, both 4:3 ratios. So why the 5:4 ratio in the medium sized monitors, and not a proper 4:3 ratio like 1280x960?
I’m not sure about why the aspect ratios came about (which is what you’re asking), but the standard sizes 640x480, 800x600, 1024x768, 1280x1024, and 1600x1200 all came about with regular CRT monitors. 1280x1024 is the only one that’s not 4:3.