mrv3000 | "Windmills do not work that way!"

I've had this random question going through my head since Last of the Time Lords aired, but keep forgetting to ask about it. 1) There probably have been discussions about it, possibly ones I've read (drink once if I even commented on it, drink twice if I had an entire comment thread going on it, drink the whole bottle if I wrote a 10 page meta on it) or 2) it possibly was answered in the episode itself, but just a whole lot blurred out for me when hitting a certain point in the episode.

All right, so why did the Toclafane kill off a tenth of the Earth's population? I mean, it seemed like it was to even things out - after killing the people there'd be 6 billion people left on Earth and 6 billion Toclafane.

WHY??

Or was the number thing just a red herring? The Master and the Toclafane were just showing who was in charge.

Did they say in the episode?