Well, you've not actually said what is using the memory. You know that the SGA is only using 9G. You've not said what size your PGA is. You've also not said how much memory is being used by non-Oracle processes.
One thing that might be tricking you is the file system cache. Most operating systems, including HP-UX use free memory as file system cache, releasing it when it is needed for something real. As a result, heavy file usage can make the system appear to be using more memory than it actually is. Most OSes give you a way to identify what is real memory usage and what are buffers for the file system cache. On Linux the command is "free". Not sure what it is in HP-UX. I guess glance will show you???