Luna Canon Quibble

I keep reading 8th year stories that include Luna Lovegood among the 8th years, despite the fact that she's in Ginny's year.  It always throws me off, and then I have to work to get myself back into the story.  At least movie-canon-but-not-book-canon things (like Draco, Ginny, and Harry having blue eyes) I kind of understand, but I don't think even the movies put Luna in their year.  Maybe it's just that the movies don't explicitly state her year?  I've only seen OotP once... 

Okay, all done being obsessive.