THere are also buildings in game that allow you to have a higher limit on a province by province basis. Royal post reduces the penalty for that province by 20% grand palace by 30% but only for that province.
So for example you can control 5 provinces given your stats.
However you have 10 provinces.
In Some of your provinces you have no buildings that effect efficency. THe efficiency in those provinces would be 50% ie you only get 50% of the normal income.
Some provinces contain a Royal post, those provinces would have efficiency of 70%
Some provinces have both Royal post and Grand Palace, those would operate at 100% efficiency.