"And how is the running of a different node different from a soft fork?"

forks have a possibility of an unintended chain split or can be used to force a chain split.

different client implementations with the same consensus rules will converge to the same chain tip.

btw you can safely run a hard fork of bitcoin. you would most likely be in consensus with the rest of the network, despite the popular retardation about "hard forks" the conditions under they can temporarily be abused are super expensive and hard to coordinate.