When it comes to application of language there are three things involved: Speaking, reading and writing.
Most people can apply it in two instances only: reading and writing. Speaking the language becomes a hard task only a few people can apply all three instances at the same time.
Some people can speak a language very well but reading and writing it is somehow difficult. Many people can speak their native languages fluently but reading or writing it is a herculian task.
Some people can read and write a language but cant speak it fluently. Example is here in kenya where a bigger population can read and write English but cant speak it very well.
Being a native speaker of a language doesnt mean you know everything about it or can apply it well than others. While I was in high school we had some students who were from the coast and would speak very fluent Swahili but the ironical part is that they would fail terribly in the exams. None of them would even top the Swahili exam papers.
Being a native speaker doesnt mean also you can make a good teacher of the language. Teaching a language and its application is very different from speaking it.