My DD did not meet the early expressive language milestones of levels 3 & up, but she'd caught up by 2 (and I'd even say that she was level 4 or 5 by 2 in terms of speech). She just exploded. In DD's case, I am certain that the reason she didn't talk early was a fine motor skill issue. In terms of receptive language, she was very, very advanced, and she was obviously trying to talk very, very early, but she couldn't always get the right sounds out at the right time. She has also not met the early-puzzling milestones for levels 3 & up. Other than that, she is clearly a level 4 or 5 kid.
I think some variation is expected. And it'd be the rare kid who fit neatly within any of the levels. You'll note that Ruf uses words like "most" and "many," which indicates that some of the kids that she put in that level did not meet that milestone.
