Considering the time and distance separation between UK, USA/Canada and OZ/NZ, it's more astounding that the languages remain as similar as they do. It's testimony to the effectiveness of communications (letter, newspaper, telegraph, radio and now internet) in preserving the understandability of the various dialects. Were it not for this constant flow of communication, we wouldn't be able to understand each other despite the common heritage.
It also helps that English has a fanatical hunger for new words.* We're conditioned to pick up any nuance that floats across our sights. Calling a trunk a boot is just so intellectually delicious that we want to remember it along with all the other variants that we encounter. Where some languages create committees to preserve purity (think Acadamie Francaise), we enjoy mixing everything up into a verbal mulligatawny (itself a word cribbed from the Indians IIRC).
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*The problem with defending the purity of the English language is that
English is about as pure as a cribhouse whore. We don't just borrow
words; on occasion, English has pursued other languages down alleyways
to beat them unconscious and rifle their pockets for new vocabulary.