Question about the characters with the same actors-Please help!

I'm confused on one part. I haven't read the book so I'm only going on the movie. How come Ioan, Tim and Natasha played both the characters in the storybook and the characters that were in "real life" with Maria? Are we supposed to believe they are the same ppl who have lived hundreds of years or are they just supposed to be descendents who happen to look like them?