I don't know for certain either, however...
I always throught "10% surrender bonus" means the ships are 10% more likely to surrender to you without a fight. I believe it has nothing to do with the amount of plunder you get. Basicly, this just means your fame (or infamy
) is so widespread that some people will just surrender when they see your flags.
The +1, -1, I belive, operate within the 0-5 range. At least I've never seen a skill 6 or a skill -1. Maybe those exist, but just aren't shown, since the bar scale is only from zero to five. BUT I belive the +1 skill can take an unskilled captive to a skill of 4. At least I think I've had unskilled captive with skill level 4 before.
Sorry I'm so uncertain about everything. It's been a long while since I last played that game.