Why jn:parse-json and parse-json give different results

Question

I have following command :

echo '{"a":null}' | xidel -se '
  jn:parse-json($raw)/join((exists(a),a)),
     parse-json($raw)/join((exists(a),a))
'

the output is

true null
false

I know jn:parse-json is JSONiq extension.

Why are the results different ?

Is there a way to get internal representation of jn:parse-json($raw) and parse-json($raw), in order to understand difference in behaviour?