The author says each of the big 5 is on a "sliding scale". I admit this is a bit pedantic, but isn't each just on a plain old scale? Wouldn't a sliding scale imply that one score could affect how another was graded?
Not to me; I am actually not familiar with any clear meaning for the phrase "sliding scale". It's just a thing people sometimes say.
If you wanted to say that your score on one axis could affect how your answers to questions on another axis are interpreted, you'd need to call them something like "non-independent". Not much call for that kind of terminology.
Can you provide any examples of someone using "sliding scale" in the sense you suggest?
I learned the term WRT insulin for diabetes. I think the key is that the scale you use for determining something (fees, insulin dose) changes depending on other characteristics of the person