MtheGM, I doubt the system purports to know with 100% certainty what you are doing, but it knows what your Web browser is doing. If there's an instance of a browser being opened to the main forum page, it counts as an idler. If it's opened to a specific thread, not just the main forum page, it counts as a reader. And if there's an open dialog box with unsubmitted content, it counts as a writer.

In either of the above cases, what the individual running the Web browser is actually doing may be somethng else entirely. He may have paused in the middle of writing a message and gone out for a beer. But the system doesn't know that so it counts him as a writer, not a drinker.