At least a part of the "used" memory might actually serve as a disk cache that will be flushed once an application needs more memory.

NOTE - What is disk cache: As reading from RAM is faster than reading from persistent storage (still true even for modern solid state drives, as the one used in the N9) the OS might decide the cache data when it gets reading request so that subsequent reading requests are served directly from memory, not from persistent storage. It might even read a bit "around" the files it gets a reading request for. Result examples: Launching an application for the second time should be faster, working with a collection of files should be faster once the first file & surrounding data is cached in RAM.

NOTE - Free RAM: It sensible to use all available RAM, even if there are no applications needing it at the moment - it still needs to be powered, even if not being used at the moment. Caches are good candidate for all this unused RAM as they generally can be easily flushed (they just cache data available from somewhere else for faster access) once the RAM the occupy is actually needed by applications.