OS X can only scale by powers of 2. If your monitor runs at native 4K (3840x2160), scaling ~1.28x renders to 6016x3384 and scales down by 2x to 3008x1692. Your example is at 1.5x (2560x1440), which renders to 5120x2880 and scales down by 2x to 2650x1440. The most efficient scale is 2x (1920x1080) because that can be rendered natively. So while you can have arbitrary effective resolutions, it's rather wasteful to render and can have serious performance implications when you're not using native or powers of 2.