I've made a very interesting and pertinent discovery about trade route mechanics. To summarize:
The reason our trade routes with Sian are being wasted (on cities that would get the same profit from internal routes) is because he comes before us in turn order.
The implications of this are that the later you are in the turn order, the more the trade route mechanics will screw you! Here's how it works:
First, our cities are ordered by their trade route modifier. Cities with ToA or harbors would get bumped to the front, cities with pop over 10 get a slight edge, and tiebreaker is the city order (generally the order of founding). Being on an island is irrelevant for the sorting (even though it's liable to cause overseas bonuses) because we don't know where the trade route is leading at this stage. Overseas-ness is relative.
Second, the game looks at the first city in our list of cities (which is going to be our highest pop city right now - size 12 Callisto, beating out size 12 Io due to founding order) and assigns it the best possible trade routes. To do this, it loops through all the players in player order with no special case made for ourselves. So Callisto gets Sian's two island cities, because Sian comes before Yuris in turn order. Io gets next pick and takes the last available overseas route, from Yuris, and then gets the route to another of Sian's cities.
This is just a $2 route like one of our internal routes, but it goes to Sian's city because he comes before us in the turn order, so the game saw that his city was tied for best first.
This is a ridiculous bug. I guess we're lucky we're going second in this game. I'm inclined to play with a mod that fixes this in future games (not RB mod though).
The reason our trade routes with Sian are being wasted (on cities that would get the same profit from internal routes) is because he comes before us in turn order.
The implications of this are that the later you are in the turn order, the more the trade route mechanics will screw you! Here's how it works:
First, our cities are ordered by their trade route modifier. Cities with ToA or harbors would get bumped to the front, cities with pop over 10 get a slight edge, and tiebreaker is the city order (generally the order of founding). Being on an island is irrelevant for the sorting (even though it's liable to cause overseas bonuses) because we don't know where the trade route is leading at this stage. Overseas-ness is relative.
Second, the game looks at the first city in our list of cities (which is going to be our highest pop city right now - size 12 Callisto, beating out size 12 Io due to founding order) and assigns it the best possible trade routes. To do this, it loops through all the players in player order with no special case made for ourselves. So Callisto gets Sian's two island cities, because Sian comes before Yuris in turn order. Io gets next pick and takes the last available overseas route, from Yuris, and then gets the route to another of Sian's cities.
This is just a $2 route like one of our internal routes, but it goes to Sian's city because he comes before us in the turn order, so the game saw that his city was tied for best first.
This is a ridiculous bug. I guess we're lucky we're going second in this game. I'm inclined to play with a mod that fixes this in future games (not RB mod though).