@alpaca

Come to think of it, in this particular set-up of my ongoing WIP mod, I had Milan and HRE began the game as allies with trade rights (to simulate vassal-ship). France and Milan had no such treaties.

So maybe once Milan could reach Bologna the game had Genoa open up trade with Bologna (its ally and whom it held trade rights with) instead of Toulouse. In which case, Toulouse sought out the next-best port to trade with - Rome in this instance.

I'll have to do more testing when I get home from work. But it does seem strange that Toulouse received substantially less income from the exact same exports when it was trading with Rome as opposed to Genoa, even though Rome is further away. So who knows how trade is calculated...