Apparently there is some improvement that you can build on a planet, after which ships in orbit around every planet get higher attack values.
This improvement also seems to give troop transports a 1-1-1 defense rating.
Now, I haven't been paying very close attention, so I have no clue which improvement this might be. But I noticed an error: any ship with a troop transport module always gets 1-1-1 defense in orbit. Even if this ship has a much better defense by itself, it still only gets 1-1-1 when it's in orbit around any planet. I suppose this is a bug. One of the core ship types is even an armed transport, which will also get the 1-1-1 defense when in orbit.