Secrecy

[Darth Ultron]

So, if the PCs decide to "save the town of Highrock from the orcs of Thung", not by (collecting the seven super special secret McGuffins and) fighting the orcs as the plot calls for, but instead by evacuating the town, or starting a week-long forest fire between the town and Thung, or, better yet, both, then have not the players created a plot point / affected the plot?

Well, the players are not creating anything, they are playing along the plot. And they are ''effecting'' the plot, by playing along the plot.

I'm pretty sure Darth Ultron views the plot (the one true plot to rule them all) as what he has planned. Unlike the more common idea that the plot is actually what happens.

Right, some games just rubber stamp 'plot' on 'absolutely anything that happens in the game'.

See in my experience players will pay attention to what's happening during a given game (or group of connected games) and wind up making decisions you didn't account for, thereby ending up changing what happens because you're a good GM who lets players do things you didn't expect them to do.

In my experience only poor or bad DMs are ''always surprised" by ''anything the players do".

Games are not novels - you don't decide what will happen at every stage from the start to the finish. Unless you're just running one shots with new characters every single game the PCs are going to have pasts that they can call on, and even if you only play one-shots, PCs will never do what you expect 100% of the time.

Well, you might need to check the rulebook for your RPG....but most of them with a GM do have them deciding what happens from start to finish. And while you can't predict what the players will do, that is more your own personal falling then a blanket statement for all people worldwide.