That's because your merchant at Tunis will only be
transferring trade power. What this means, is that your merchant is at that trade node and actually steering some of that trade along your chosen trade path (the dotted lines with arrows). But as the Tunis trade node is further away from you, and you likely do not own any provinces within it's trade area (click the trade view on the map to see this, the little box to the right side of politcal view [the flag you have selected above]) you will have very little trade power so you won't actually be able to steer much traffic.
You can increase your power in each node by either owning provinces within the trade area and then building certain buildings which will increase your local trade power (marketplace and docks spring to mind); or you can build light ships (which are Barques at the start of the game), group them into a fleet and then ordering them to Protect Trade where you need to select what trade node you wish protect. This will result in your ships going up and down the trade lines, it's nothing more than a visual effect in the Mediterranean really, but when you start doing it across the Atlantic you can run into pirates which is why it's very necessary to protect your trade routes.
One further thing:
I have been testing with placements of my merchants the last few days. The trade node that is in the same province as your capital city will
always collect, regardless of whether there is a merchant there or not. Get your trade nodes set up and have a look at your trade values after a month or so. Once you get a good idea of what they are at, try moving your merchant at your "home" node to a different one to try and steer - look to see if you lose much/any trade power and can capitalise on this by steering more trade towards you.
Hope that wasn't too hard to read and gets the ideas across