Try hovering over where you found the values. Trade tooltips should be fairly extensive, but ultimately answer the specific questions asked here.
In general though, a country gets (base) trade power in a trade node by
1. Trade power from provinces. Trade centers/estuaries in any of those provinces adds to this total, but it's all in total trade power from provinces.
2. Light ships protecting trade in the trade node.
3. Merchant present. (Typically only a base of 2 trade power, but in inland nodes it can be 50 when steering and so on)
4. Transfers from traders downstream (basically increased control in downstream nodes will grant a small increase in control in the upstream node)
The total trade power a country gets in the node will be calculated by the various bases taken together and increased by factors affecting them, such as mercantilism, global trade power, merchants steering collecting outside of home node and so on)
Tooltip should show all sources. (I may very well have forgotten something or made some mistake as I am taking it from memory and am away from the precious EU4 at the moment)