Well Polynesia gets the WC bonus because it can travel over oceans, but I think the 'unforeseen bonus' is due to hotels with their statue improvement giving culture which is turned into tourism, allowing Polynesia to be one of the few civs capable of really benefiting from wide cultural-victory wise.
Carthage maybe can do trade routes over mountains? This would be an unforeseen bonus, or else it refers to the free harbours - maybe these provide bonus to money from cargo ships.
Rome I'm not sure about, possibly good for a strategy where several cities boost productivity in the capital to build a building, then the trade routes swap direction and the capital helps the cities, allowing for an even faster boost to empire-wide infrastructure.