Suppose I can run some graphics in 1366x768 equally well as 1280x720, and I have a 1920x1080 monitor.
There are more pixels in 1366x768 than in 1280x720. However, in 1280x720, a 2x2 grid of pixels fits exactly within a 3x3 grid of physical pixels in my monitor.
So which should look better, and is there any theoretical basis for which one looks better? Does the 3:2 ratio mean anything at all?