AIUI, here's the pathological case:
City A is size 16 and has a market, grocer and bank. (Let's pretend you're running 100% gold for the moment). It gets a high-value trade route from a foreign city.
City B is size 12 and has nothing. It doesn't get that high-value trade route.
City B builds a harbor. It is now effectively size 18 for the trade route calculations. So, it gets that high-value foreign trade route, and the base value of that trade route is slightly higher than before. But since it's missing the +% gold improvements, the result is actually less gold.
edit: Blake has rendered this post obsolete and slightly wrong
City A is size 16 and has a market, grocer and bank. (Let's pretend you're running 100% gold for the moment). It gets a high-value trade route from a foreign city.
City B is size 12 and has nothing. It doesn't get that high-value trade route.
City B builds a harbor. It is now effectively size 18 for the trade route calculations. So, it gets that high-value foreign trade route, and the base value of that trade route is slightly higher than before. But since it's missing the +% gold improvements, the result is actually less gold.
edit: Blake has rendered this post obsolete and slightly wrong
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