A bit late in this conversation but it is something I come across all the time. When I eventually will be allowed to visit the UK again, I am certain I will have another difficult conversation with my parents about the language I am now using! But seriously, it is interesting how the language changes and how cultural changes in society has influence those changes. British English has always been a melting pot influenced by thousands of years of people immigrating into the country (sorry Brexiters, but there never has been an England for the English, especially while you munch upon your Pukka Pie).
Earlier languages which now exists in sometimes rarely used (by the masses that is), dialects or even separate languages (such as those in Cornwall, Wales, Scotland, etc.), got replaced as waves of Romans, Saxons, Vikings and Norman French came through, which is before we even start considering words and expressions we had borrowed from other cultures that have settled in the country. This is what makes British English the thing it is, both easy to pick up in a rudimentary way, yet a hard language to master, especially correctly enough to pass a GCSE. That was one of the arguments I had with my English teachers at school, because whilst I would use language that technically passed rules of grammar as taught, it was deemed incorrect, or too simplistic.
Experiments I performed upon my unwitting teachers went on to prove that actually you had to break all the rules and specifications as set out by the examinations board to achieve a decent grade. For instance, I wrote two pieces of fiction, one was effectively a rewrite of the first attempt, which was technically within the specification of being concise, on a single page and between 450-500 words. The first attempt was scored as an "E", meanwhile the second attempt was typed (that’s how I got the word count), very "wordy" ringing up over 1800 words, on three printed pages, unnecessarily expounding on irrelevant details and so forth. Oh and the early grammar checker also complained it broke all the rules (of course it didn't understand British English because at the time it greatly differed from American English syntax and grammar).
Anyway this attempt scored a "B+", which led to many complaints and refusal to be bothered with subjects such as geography and history because of the high level of marks attributed to "perfect English" than anything to do with the actual subject. Why have another grade telling me what I already knew; that is my written English is/was poor, and not of a good enough standard. Yet I was in top sets for other modern languages initially, simply because I could not cope with rules and laws that must be broken in English (I mean what sort of rule is that), yet adherence made other languages simpler to understand.
This of course is now my problem with services such as Grammarly - Technically maybe brilliant, and probably the way things should be written. Yet it is probably one of the most dangerous things to happen to English, because instead of embracing diversity, is encouraging homogeneity. However creative people can not typically be creative with homogeneity, as we see from Shakespeare.
As previously mentioned, his language is not comparable with todays, yet even at the time it was very modern, with words and language being introduced. Of course, Shakespeare did often write like a modern musician or poet with scanning in mind. He also wrote using the regional accents and dialects of the performers in mind too. So, while some of our modern productions don’t seem to make sense, they would make much more sense when read or performed with a regional accent for certain characters.
Anyway, onto the subject of American English. My understanding is that whilst yes there are differences because of the differing levels of influence from third parties, it was, in reality, a deliberate break in many ways from British English. There were a couple of reasons behind this. Firstly, at the time, it was deemed that America should shrug off its colonial roots, and develop its own culture, language, etc. By the time the ubiquitous Merriam-Webster dictionary of American English was being compiled, new words and phrases were entering into the North American Language. I mean, how many skunks did we have in Britain, as I am sure that Sir Francis Drake or Sir Walter Raleigh might have overlooked bringing one over for Queen Elizabeth to gawk at, a couple of centuries earlier.
Then if we are creating a new dictionary, we could perhaps change the written form of some words from their traditionally French/Latin written form to how they are pronounced in English. This in theory should make spelling easier for those trying to learn the language, because they grew up with a different first language (I don't know, let’s say migrants from all over the world; see kids nothing is new, immigration has been a hot issue since the start of the USA). So we get changes such as colour becoming color, centre becoming center, metre becoming meter. Some of these changes were also getting exported back into the mainline British English too. Yet some are, maybe by urban myth, also accidentally introduced into the language, such as how aluminium became aluminum.
So having been a very frequent visitor, and now living and working in the USA, I am now suitably proficient in both forms of English, but also find myself often being misunderstood in both the UK and here in the US. For instance, just the other day, I was in the cafe, talking with the servers, saying I couldn't understand why the business had slowed down because I had not heard any complaints, "not even a dicky bird". Of course, that led to rapturous amounts of laughter, and requests of "can you please say that again" with an explanation of what I meant! Then a few weeks later, talking to my parents on how we need to change some of our data network because of a cutover to a new router (pronounced the American way), due to fiber being installed, got a response of "huh". Not because my parents don't understand IT (they ran an IT consultancy themselves), but because of the language differences even with modern tech.
Anyway it is always very interesting and sometimes very annoying, especially when I have to get the kids to say something on my behalf because the teenager at the drive through has never heard a real Brit before!????????