only losing one man in the effort |
Remember you're also losing a transport that cost billions of credits to build. And no matter how good your economy is, you're also losing the real time that it took you to select the option to build that transport, launch it, click the destination planet, wait for it to get there, and click through three invasion screens to kill a few hundred aliens. Not saying it's bad, just that there are a lot of resources expended for the effect.
As far as those rocks not hitting the planet, but the invader still getting his bonus; the way I see it, a planet has a certain amount of both ground and orbital defense that the game rationalizes into one defense factor. A big part of that defense is taken up fending off your bombardment, leaving the assaulting troops a bigger attack bonus.
When the attacker loses, that means he didn't use enough troops to swamp the orbital defenses. Those stayed intact and were used against boulders instead of the attacking troops' drop pods. The ground defenses were still strong enough to keep the attacking soldiers away from the orbital defense installations and the entire attack was foiled for lack of troops.
It's not a direct correlation, but if you can find a copy of Robert Heinlein's Starship Troopers (the book, not the braindead Hollywood version), read the chapter about Operation Bughouse. It talks about a lot of things that can go wrong in a planetary assault; kind of like D-Day with Vietnam-era rules of engagement, or if we hadn't had Italy to practice on first.