I found some unused icons in the game, including a "mantle excavator" and a "core tap."  These got me thinking about a race that is so evil, they don't even think twice about destroying the planets they live on.

If a central mine were to cause a 10% loss in planet quality, and a mantle excavator were to cause a 20% loss, would the planet suffer a total of 30% when the central mine upgraded to a mantle excavator?  Upgrading to a core tap, with a penalty of 30% would leave most of the planet uninhabitable.

If each version caused a 10% loss, the planet would suffer a cumulative penalty of 30%, but those planets untouched by a central mine or mantle excavator would only suffer a 10% penalty.  That just wouldn't be right.

I'm under the impression that the designers of this game were thinking along the same lines, but couldn't square that circle, either.