The extra gold comes from a variety of different sources and the total shown in the trade node isn't accurate to what you'll bring in.

Go to your various trade routes and mouse-over the gold income down on the list amongst the other people in the trade route to see what your bonuses are.

For example in my Hungary game I loaded up to try answering you-

In Constantinople I would originally earn 10.15 gold per month for having 74% of the trade route's ~13.4gpm. This 10.15gpm is boosted by 20% from 'technology', 15% from Burghers, 20% from being ahead in Diplo tech, and 10% for trade efficiency advisor for a total of 65% bonus to my GPM. This brings my GPM from constantinople to 16.76.

Another thing to keep in mind if you didn't know it already is you take a huge hit in trade power in a trade node if you collect there without it being your main trade node. Something like 60-70%? Similarly you get stacking bonuses towards your trade power in each node along the way when you transfer trade power.