DD6 took a big leap forward in reading, and out of curiosity I looked up the level of some of the books. According to this site, they range from late 2nd grade through 5th grade. How meaningful is this?

Can a few difficult words bump a book up to the higher grade levels? (We're using advanced picture books rather than Easy Readers, hence not a carefully controlled vocabulary climate.) Also, the "interest level" ratings on this site are really whack, which makes me doubt the whole thing.