This is another guess,
The Spawn Nodes might be getting populated in a sequential fashion.
For example in Kiel, at the start of a campaign, the Schleswig_Holstein is due to sail shortly, and it is always placed in the spawn node called NodeP
1961. Which is the lowest numbered (1961 in this case) spawn node, inside the range of the Generator Node that will generate it
Now the reason for that could be that the NodeP1961 comes before the rest in the mission layer file (.mis), or the name is interpreted to achieve this ordering.
Edit: Alright I tested this thoroughly and it's the actual position of the Node definition in the file. NOT the NodeP1960 or NodeP1961 names. So if you add your own Spawn Node to the project it will come last and thus get populated last.
Anyway this might be not that important, just putting it out there for the knowledge base.. It also seems warships will be picked first, and then the merchants, for populating these so called
spawn nodes.