I once had a Spanish teacher who said we were allowed to ask any question in class about the language, except "Why?" He proceeded to present to us a lot of "rules" in the English language that made no sense unless you were a native speaker, and even then a native speaker wouldn't necessarily be able to explain why it was the way it was.
Case in point, the differences between American English and British English.
There is a more subtle difference that I have been noticing lately in the usage of -t, instead of -ed at the end of words. I say more subtle, because there are past tenses of words in American English that end in -t, but British English seems to use it more often, and on words I would typically opt for an -ed ending. (e.g. Learned-learnt, spelled-spelt)
If I go into the grammar, the languages tend to agree on using a -t if the word is an adjective, like 'burnt toast', but then the similarities seem to come down to personal preference. The one that makes me go back and re-read the sentence most often is 'spelled' vs 'spelt'. I think of spelt as a grain, and it doesn't matter if the writer wasn't even talking about food, my mind always stops and says, "wait, what does spelt have to do with it?"