Why are we getting more retainers?

[Xieveral]

4) less issue to deal with the delivery tax they have for buying from retainers in other city-states.

I have only seen the tax discount once and that was during open beta. Ever since, I have seen it at the normal rate for every city. What is it that affects tax rate? I have heard that it depends on the number of retainers dispatched to a city.