Wondering how poeple sound different

I took a nap after I woke up and already had breakfast this morning. That’s because I felt tired. It’s most probably from yesterday’s TOEIC Test. Now I’m wondering how the pronunciation differs from person to person. I know it is different from country to country. British accent has its own features, so does American accent. But what I meant here is about more individualistic aspect of English pronunciation. There are some British people I can easily hear and understand, but on the other hand, there are other British people I have some trouble understanding. I cannot understand every word of them but some details. That was what I encountered when I was listening to the listening section of TOEIC yesterday.