If RAM gets critically low, running background processes can be killed too and they are part of that count listed above with the available RAM. However, this only happens after all other cached background processes have been killed first.
It seemed with Android 2.1 and earlier, that if you left your handset on for weeks and weeks, eventually the RAM would start to get fragmented and then only a couple of processes could be cached when multitasking. So, in that event, every time one opened a RAM heavy app like the web browser, it would kick the cached HTC Sense process out of RAM when one went back to the home screen, and it would have to reload and vice versa if one went back to the web browser. I think that Android 2.2 and Android 2.3 mitigated this with more efficient memory management and better garbage collection, thus reducing RAM fragmentation. Handsets and tablets today have so much RAM that this is hardly an issue anymore anyway.