"The difference between buildings and settlements is one of scale."
Does the phrase 'one of scale' mean 'the only difference is'? And so there is no other difference?
I'm not quite sure why you are puzzled by this, perhaps it's the word "one". In "one of scale", "one" doesn't mean that it is the single or only difference, "one" is a pronoun referring to the previously mentioned noun, "difference". To rephrase it: "The difference between buildings and settlements is a difference of scale".
I hope that helps.
not a teacher