Karel Jansens is right. It is a feature of Linux (and many operating systems) that any file read from the file system is kept in memory for as long as possible, in case it needs to be accessed again. This is because file system access is (generally) much slower than RAM access.

"Cached" memory is basically free memory, though, since it has very low priority and will be trumped if applications need that memory for other purposes.

The load applet is aware of this and treats Cached memory as free memory.