Reality in Fantasy

As I've been reading Anathema, an interesting question has come up. How real does fantasy have to be? For instance, Anathema, is set in a world make up by the author, but it resembles a middle ages type town. The reason I bring up realism is because often the speech patterns of the characters in this book are not very consistent. At times the characters speak like any villager might, but at other times-sometimes within the same paragraph-more modern speech patterns slip in. Do inconsistencies like this in fantasy bug anyone else? Personally, I think even in fantasy, the setting, dress, speech, and attitudes need to be as realistic as possible. Create your world and stay true to it.