As a game that requires high computing power, it is expected that the mentioned computing power must go somewhere. One of the things it will go to is calculating the variables of every state in regards to the building it can build and sustain.
As mentioned all the way back in the dev diary about Buildings, States are limited in which Resource Industry Building Types they can support and how large these can get. For example, the degree of Iron deposits in a state limits how many levels of Iron Mine you can build there. These resource limits are actually a property of State Regions, which is dynamically allocated to the States in the region based on how large of a share the State holds.
It’s worth noting that this proportion isn’t just based on the raw number of provinces owned. As an example, the potential for Fisheries is distributed according to the amount of coastline a State has in the State Region, while Arable Land land can be heavily weighted by the amount of Prime Land is in each State.
While not applicable everywhere, Prime Land is something used in states where there is a clear division between fertile and non-fertile land - control of the Nile should matter a lot more to a State in Upper Egypt’s Arable Land than control of the surrounding desert, for example.
Paradox Interactive
It’s also possible for a State Region to contain Discoverable Resources. These are resources such as Oil, Gold, or Rubber that are either not known about or not considered exploitable at the start of the game but may be discovered and exploited at a later point.
Certain technologies will affect both which resources can be discovered and the actual chance of said resource being discovered. All of this functions in a weighted random fashion, so while the chance of there being a Klondike gold rush at some point during the game is high, it probably won’t happen exactly at the same date it did historically.
