Its entirely subjective. My scale is not the same as my friend's scale and his isn't the same as my cousin's scale. We all value different things in different quantities, and that defines where they go on our scale. You really shouldn't worry about it.
2 girls I know come to mind. One is an 8 because she has pretty blue eyes. The other has big eyes. Sometimes it's just a quality a girl may have. I know a 8 1/2 but has the personality of a double zero. So she's really a ?