At one job I ended up having a lot of work writing language bindings from C to what is commonly considered a "scripting language" (TCL), so it's easy to have to deal with memory management and low level systems knowledge in relation to a scripting language (knowing the semantics of memory management within a language drastically changes how you have to write the bindings). However, the idea is to rate your skill with that particular language. No matter what the most advanced state is of a language, a 5/5 (or whatever the scale is) means that you can accomplish the most advanced task possible in the language. That doesn't mean that someone who is, say, 5/5 in Perl, will have the same skillset and capabilities as someone who is a 5/5 in AT&T syntax assembler. However, there are a list of base assumptions of skills they do have that may align with what we need as a developer when looking at resumes.