The problem with these burial sites is that a lot of them are in the grounds of churches and over the years the buildings fall into disrepair and no-one takes charge of the records which are sometimes stored inside the buildings. I read about this kind of stuff in a book once a long time ago.
Also if you go back far enough, people were buried in plots on their family's land and a lot of those records would go missing. You just never know what's underneath your house.