i've seen that too on conquered planets. not really sure what it's supposed to mean. the "improvements lost" and "sectors lost" statistics are a bit confusing to me.
i think GC2 had similar mechanics, so i figured that "improvement lost" means that some factories got blown up during the ground combat, so the building is gone but the tile is still there. "sectors lost" means that some tiles are unusable. the conventional ground combat doesn't seem to destroy sectors, though. that's only when you play the "bombardment" card that represents your guys throwing asteroids at the defenders.
there may be a bug in that invasion code. i'm pretty sure i've seen such "ghost tiles" on planets i conquered without destroying sectors.
or maybe i'm just misunderstanding the two statistics and they mean something different.
