At least I THINK it's a bug.
I've noticed that when I send ships back to planets for repair, if the amount of damage is not a multiple of the planet's repair capability then the ships are actually repaired for more than their maximum hitpoints.
Example: I have a ship with 7 points of damage at a planet. The planet repairs 3 points a turn. After two turns my ship has only one point of damage. After the next turn, instead of repairing just the one point, the planet goes ahead and heals another 3 points, giving my ship an extra two hitpoints.
Also I seem to be able to repair ships at planets without a starport. Don't know if this is intentional or not.
BTW what determines a planet's repair capacity anyway?