why is Mark Twain easy to read but difficult to understand?

why is Mark Twain easy to read but difficult to understand? why is Mark Twain's works difficult to read? why is BBC so difficult to understand? why is American humour difficult to understand? why is "pride and prejudice" so difficult to understand? English is a language easy to learn but difficult 翻译：Mark Twain is someone Americans are taught to love. It is easy to learn English ,but it is difficult to learn it 英语翻译Mark Twain,Who wrote the story we're going to read,trave 英语翻译Mark Twain who wrote the story we are going to read trav which letter is difficult to understand Which letter is difficult to understand?