Natives are not necessarily proficient in their language
Even a true native, whose family has been for millennia in a certain country, may not speak or write correctly. If they belong to an uneducated family, if they don't read books, if they did not pay attention at school, if they belong to the computer games generation...
When working as a journalist, at some point I was responsible for reading and dealing with readers' letters. (My mother had a page where she gave advice to readers on various matters and I was the one to do the job behind the scenes). I was appalled at the level of ignorance those - presumably native - people had of their own language. Especially the young ones. You immediately saw the difference between the younger and the 35+.
Even the appearance of those letters: few or no accents, wrong punctuation, sometimes no margins, written with a red or light blue pen, mechanical unjoined handwriting which mimicked printed fonts...
I'm pretty sure that it happens in most countries.
From 1998 till now I've been reading internet comments by native English speakers on various fora, groups, youtube, disqus etc, and it seems that writing correctly is very low in people's list of priorities.
The level of education is not what it was
(you can skip this rant if you like and go to the next point)
My children and I went to the very same school (Athens Italian School), and I could witness first-hand the abysmal difference in the quality of teaching and in the expected proficiency.
For school assignments, most of the students copy pasted chunks of text from the web hardly reading it, forget about understanding or changing it. Most of them spent class time on their cellphones and, on coming home, turned on their PC screen and started to play. OK, not everyone (in every class there was someone who got top grades by studying hard), but a shocking majority.
With no real consequences. Because nowadays it is considered "bad form" for teachers to make you repeat the class, and parents protest very vehemently if it happens. For this to happen, a student has to really really show he doesn't care a hoot and do strictly nothing all year. In my times you could be made to repeat the class even in elementary school. Now the worst that can happen is that they are made to repeat the exam in September for certain subjects, and in September teachers are mostly lenient, even if the student just took the time to study for the last ten days of August. It's very rare that someone really doesn't make it. With the result that more ignorant and irresponsible students go on to the next class, not even having the basics, thus understanding even less of the new stuff, which of course discourages them even further from making an effort.
I also saw the difference in the Cambridge English exams, the Lower (now FC) and Proficiency, as well in the Sorbonne exams. It has become so shockingly easy nowadays! Presumably not to discourage people and get more applicants thus more money.
In my time, you actually had to write. A lot. Now it's mostly multiple-choice questions, you just have to tick a box. Instead of studying a whole literary movement with focus on a certain author or group of authors, with lots of reference books "about" them included, nowadays you only have to read ONE single book, with no context. And, in Cambridge Proficiency, you don't even have to read the book. There are two questions, one about the book and one about other things, and you can choose one or the other.
Let me not even touch the state of Italian and Greek universities, where half of the time is spent on protests, strikes (either of students or professors) and... how do you call them in English, "takeovers"? When they occupy the building and no lessons can be held, and inside the building there are sex and booze orgies alternating with political meetings and burning of furniture bought with taxpayers' money?
Those are the people to whom tomorrow we will entrust our health and legal matters, the safety of our buildings, the running of our country.
Knowing all this, still - silly me - I was surprised by seeing, here at viki, how little people know their own language. I'm speaking of the Greek and Italian teams, which are made of "real" Greeks and "real" Italians, born there, of native parents and grandparents, who went to local schools.
English teams are much less homogeneous, members can be from anywhere in the world. Including some who may technically be 'natives' because they were born and raised in an English-speaking country (such as the U.S.A.) but in reality second- or third-generation immigrants whose parents' English is questionable. Thus they never heard proper English at home while growing up and well, at school, their classmates could very well have been of the same type of background, or "real natives" but of uneducated parents, so...
My point is...
Unsurprisingly, some non-native English speakers can have a better level than natives, because they actually studied the language diligently and enthusiastically for many years, not taking their knowledge for granted.