The game advices to set the DC by the difficulty of the task, not by it's relevance to the 'plot'. Granted, it doesn't go into much detail in this and it should. But I generally feel DMG is rather lacking in useful advice department.Well, I have no idea where the Scottsmen came from in all this, lol. I think the game surely doesn't give you any OTHER criteria to use EXCEPT railroady ones.
OK. So what would this sort of mechanic actually look like? What games use such mechanics?As for answering your question, I think in order for something to be a simulation, in any meaningful sense, it must take in all the most substantial relevant factors, and it must produce results that match with what is being simulated. There is a mapping which must exist Reality <-> Simulation in which you can say "if I run this simulation, and it produces such-and-such results, that tells me something about the reality it is mapped onto." Otherwise the term 'simulation' is literally without meaning. Every single process of any kind can be termed a 'simulation' of something! I gave the example of the wall. What is the condition of the wall? If that isn't feeding into the model of climbing the wall, then I don't see how the model can be a simulation of anything. I mean, sure, the exact subtype of limestone the wall is made of may be a factor so minor we need not consider it and we can still get meaningful results. However, if I go out with actual people and climb actual walls, the simulation needs to tell me something about what to expect, or it is not a simulation.