Right now we add the "MemFree" lines and "Cached" lines together -- I think we should also add the "Buffered" line.

For example, this is what my machine's memory usage looked like: The blue is the "Buffered" memory taking a bunch of space

This might be because I had processes writing a large volume of data to a disk, a rotational drive, even, and then, for all I know, Linux keeps those file pages in the buffered category, not the page cache category, until the page gets accessed again.