Remember that you don't get as much credit for a resource if you have a closer resource available of the same type in a province you own. Venice might be a good example of this. There is a gold source much closer to Venice, just to the northeast, which would detract from the value of the Timbuktu and Arduin gold spots. Likewise, you can make good money off of the silk at Baghdad, unless you own Constantinople and Nicaea, with their own silk.