I think - as has been pointed out before - that these monitors with 1280x1024 resolution do actually have close to square pixels and a 5:4 ratio, even when they state 4:3. I think 4:3 is simply used as a common term for "old-style" proportions, as in a monitor is described as either 4:3 or 16:9, even if it's actually 5:4 or 16:10...
