One thing to remember is that none of the tests being applied worldwide are actually a test of your English skills, but your AVIATION English skills, so it is perfectly feasible for a native to score less than 6.
Being English does not automatically confer level 6, and why should it? My grandson speaks excellent English, is he automatically a level 6? There are many other tests you can take to demonstrate your competence in the English language generally, but the DGAC test being used to grade ICAO levels is testing your ability to communicate, in English, in an aviation environment.
How good the individual tests are, well that's an entirely different subject
TTFN