After I moved to Australia, I changed the computer language to Australian English and my Mac autocorrected everything for me. It's handy when I need to write essays and business documents.
I always use American spelling online, but I didn't bother to change the settings or manually correct the spelling.
So I hope this explains something.
Actually Bruno might be onto something. It kinda seems your English got a million times better overnight, and I am talking about sentence construction and structure, not just grammar.