I agree with a lot of what Carlos said. English (including American English, Australian English, and the host of countries (India/China/Japan/Denmark/The Netherlands etc) who speak English as a second language right from the ages of 4 or 5) is a constantly evolving language. I speak very differently from my parents, the kids at school talk in ways (and not just using 'new' words) which make me sound very different from them.
Some of it will be stylistic choices, such as in speech, because not everyone has the same level of education/grammar/speaking style, and that helps to make character different and distinct in the way they talk
Sometimes it will just be little things which weren't picked up in editing. From personal experience, it is impossible to catch all the slip up's in a long story without at least 3 people doing 3 passes each. And even then... the Fifth Harry Potter book rather famously contains a scene where someone is seen in the woods who died three chapters previously, because the fact JK Rowling had typed the wrong name was missed by every single editor and test reader - and that's for a multinational publishing corporation!
Sometimes we must just accept there will be tiny errors in fiction people put out for free for our enjoyment, and they're probably fine with the fact it's not 100% absolutely perfect.