Question Alienware AW2721D - "Custom resolution not supported by display" (?)

[hideous please]

I frequently use custom resolutions on this monitor and have never experienced any problems doing so. However, recently I attempted to create the custom resolution "2400x1440@240hz" and was told that my display doesn't support it. I have no idea why it's not supported though, it doesn't go above the native 2560x1440@240hz resolution or it's pixel clock. Seemingly that'd mean the custom resolution is supported by display right? I am naive to this subject so I'd love to be educated.