Although your device may have 512 MB of RAM in it, the settings almost always report the RAM that is available to user processes, not all of the physical RAM. This means that it won't report any memory that is being used by:
- Android's system-level processes
- Your GPU, which often uses shared memory
- Any memory a cellular radio chip may need to function (irrelevant in your case)
Although this memory is not reported, it is being used by the system. Reporting it in a system monitor is simply not very relevant because the OS is going to basically keep it for itself forever (after all, the OS needs RAM to run properly, too). There is really no way that you can get this RAM "back" from the system, but you wouldn't want to anyway since your phone would basically stop functioning without it.