The Trouble With Double Yellow

My daughter's school uses the Fountas and Pinnell system of leveled readers.  The books (and children!) are rated on a scale of A (easiest) to Z.  Presumably to avoid competition and feelings of inadequacy, the school marks the books with colored stickers rather than the letters.  Of course, the kids figure out the meaning of the stickers right away!

My daughter's teacher has her reading at Level J (or, "double yellow" in sticker parlance), which are heavy on Cynthia Rylant's Henry and Mudge and Mr. Putter and Tabby books.  When she recently told me that the double yellow books are too easy for her, we decided she'd ask her teacher if she could try harder books.  But before she had a chance to do so, she came home with Mirette on the High Wire by Emily Arnold McCully, marked with the same double yellow stickers.

Mirette is a far cry from Henry and Mudge.  My daughter can read 95% or more of the individual words in Henry and Mudge and understands them all.  In Mirette, though, she was faced with words she either couldn't pronounce or didn't know the meaning of - or both!  And while she is pretty good at figuring out the meaning of words from context, these had her stumped, and with good reason.  Mirette contains words like vagabond and widow and boardinghouse and phrases like "traveling players."  Not something your average New York City first-grader is familiar with and not exactly in the same class as Henry and his big dog Mudge!  While it is great to read about unfamiliar things, this book is completely impenetrable to my daughter on her own.  It is far from a "just right" book, as books that are supposedly on her level should be.

While the contrast between the difficulty of Henry and Mudge and Mirette is dramatic, it's not the first time I've noticed inconsistencies among books which are ostensibly at the same level.  No leveling system can be perfect but, given these inconsistencies, how useful are these systems?  Are they popular now because they are yet another way to "objectively" evaluate student (and, by erroneous extension, in my opinion, teacher) progress?  They are helpful as general guidelines, I think, but not beyond that. 

What is your experience with leveled readers?