the typical Harry Potter novel is as difficult as Moby Dick! When contradictions exist, examine your premises.
I couldn't read Moby Dick until I was past 35, and even then I needed to keep a dictionary close at hand.
Something is wrong with the methods you chose for scoring.
ML/NJ
They are just automated scores based on simple measures of word length, number of syllables, sentence length.
The SMOG Index is as follows:
The Flesch-Kincaid Grade Level is:
That you needed a dictionary for Moby Dick is understandable because of the archaic words used. Those words would have been familiar to the reader at the time of publication. Likewise a reader of the 1850's would need a dictionary to read Harry Potter, if magically a copy of one of those books could be sent back in time.