Well you only seem to be missing 12MB, right.......that could easily be taken by background applications. Remember that with 9.1 ( and OS X too ) more and more OS "features" are treated just like applications, and will grab small amounts of ram if you have them enabled.
But as for opening more and more apps, that will cause memory fragmentation, which will leave small gaps (wasted, and unusable) in between the blocks of free memory. You can only get this back by quitting the apps in the reverse order that you opened them, or restarting your computer.
Turning on VM will reduce the fragmentation somewhat, and allow for more accurate mapping of the memory that is used, thereby making it APPEAR that you have more memory available than you really do.
Just be sure to only set VM to 1MB above your real ram amount......ie 192MB real ram, set VM to 193MB. This will allow for the above process to happen, but will allow only minimal disk swap file usage, which is all VM actually is, using space on the HD as fake ram ....booooohhhh........
With ram prices so low, now would be a good time to stock up if you can possibly afford to.......get it while it's cheap