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It's about the aspect ratio. The screen itself would be filled with a 0.4MP camera (yes, that's zero point four). However the screen aspect ratio is a little weird compared to a normal shooting aspect ratio so they default to 6MP so you don't need to zoom to fill the screen.
If you display your pictures online it really doesn't matter what you shoot at, a typical on-screen display is probably going to be about the same 0.4MP as the X's screen, even really high resolution desktop wallpaper at 2560x1600 is only 4MP, and most of the time you'll be at 1280x1024 which is 1.3MP. No matter how you slice it, in the online world most of the X's pixels are wasted.
If you print that's a different story, there you'll need to understand more about the aspect ratios about the print formats but even at 6MP resolution you're able to print at 10.9"x6.13" at the generally accepted good dot pitch of 300dpi and twice that size at the still good quality 150dpi - well beyond the 3"x5" of most holiday snaps.