Language is not a static phenomenon; it is constantly changing. That is one of the reasons we cannot count the number of words in any language. It is also why we have dialects and why we are not always sure which is the right way to say something.

Although languages are passed on from generation to generation, they are passed on imperfectly and with every new generation they change in very small ways. Over periods of hundreds of years can we actually map these changes and see them clearly, but they are happening right now in every language on earth. Our difficulty in deciding whether to say "an historical" or "a historical" is evidence of that fact.