As a foreigner, I can try to answer that. I've noticed that in general foreigners have a much better grammar than English native speakers because we (at least in my case) went through boring English classes. Learning English was a conscious effort and every grammar rule had to be learned and understood.
Reading English also requires a conscious effort, and reading poorly written English requires an even greater effort to parse the bad and incorrect clauses, something a native English speaker doesn't have to do.
> I've noticed that in general foreigners have a much better grammar than English native speakers
I have to disagree strongly - not my experience at all, with Indians, Chinese, Germans, Spaniards, Belorussians.. One exception, a Greek who could kick my verbal ass. But, of course, native speakers have the huge advantage of proper use being just intuitive but, still, you made an absolute statement.
You may be interested to know that when a conquered people adopts the conquerors' language, they take on the vocabulary but much of the grammar of the old language remains. I read an interesting paper about some grammatical trick you could do in Germanic languages, but not in English. Why? Maybe because, in this way, Celtic was shining through? No, non-British-aisles Celtic has the same trick. The paper speculated that it was a remnant of the grammar of pre-Celtic, pre-historic Briton. (Just to illustrate how hard it is to adopt a new grammar.)
Also, note, general foreigner, it's 'much better grammar' not '/a/ much better grammar'.
Reading English also requires a conscious effort, and reading poorly written English requires an even greater effort to parse the bad and incorrect clauses, something a native English speaker doesn't have to do.