All languages drift, begin pertaining to different grammar, and adopt
different spelling. That's why English isn't German, and that's why
Americans spell things differently than English people do. And that's why
people in Canada don't sound like they are from New Zealand.
Furthermore, the time at which languages split into different dialects and
the other languages with which they come in contact has a great influence.
And it isn't like English people have 100% perfect grammar. Universities
have done studies on how fluently different areas speak English and how
well they obey grammar (in relation to common grammar rules of the
area), and the Pacific Northwest has been found to speak English the most
fluently. So the most fluent English is spoken in America and Canada;
however, that's not to say that there aren't people who pertain to
grammar well in other countries that mainly speak English. Nonetheless,
my cousins from England use atrocious grammar a majority of the time.
But still, I'm from Seattle and I spell 'theatre' not 'theater' because it looks
right with an 're' ending.