Rince
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I think this was done using an advanced map editor. In the regular editor that I use, you can see the minuses, like in my screenshot, but I don't know how they were set. And for some reason, in the game itself, every week it displays an increase of 65,000 creatures.
It's funny, but you can do this in the standard editor. Just type a negative number in any text and copy it into the input field in the properties of the message timer.
It's strange that no one but the author of this map came up with this.