RX100: Why the ISO6400 limit?

I'm incredibly impressed with what this little cam can do in dim light. Images taken at ISO6400 are very usable ... excellent retention of detail and not much noise. Check out the shots of a currency bill taken in a room with the curtains drawn (incl a 100% crop). Other shot also at ISO6400 (max), and a portrait at ISO2500. All in-camera jpegs with NR set to minimum. Even better detail if you do RAW conversion (but to Sony's credit, the difference isn't huge).

If the camera can perform like this at ISO6400, why not give us even higher ISO settings if we need them 'in a pinch'?

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NEX-7, Sony a900; SEL18-200mm, CZ135mm, CZ24-70mm, SAL70-300mmG, 50mm F1.4, 100mm F2.8 macro, KM17-35mm, CZ16-80mm, KM11-18mm, SAL18-250mm