There are several possibilities for this observation, IMO:
1) child isn't being assessed at a level commensurate with ability. (So she's at the ceiling-- and was last marking period, too)
2) child has learned that to fit in, she needs to act like other children, and is choosing reading material that makes her fit in better with peers whose readiness level is lower than hers,
3) child doesn't have any access to reading materials at her readiness level,
4) child has discovered that higher level reading materials which ARE available to her do not have the same interest level for her, given her age.
Of those, only 4) is really something not to worry about, in my experience. The others all indicate some degree of poor fit educationally, and are worrisome.
I've definitely observed factors 1, 2, and 4 with my daughter.
There is a way to mitigate item 4; go backwards in time and choose children's literature with publication dates in the 1960's or earlier. There, reading levels up to lexiles as high as 1000 or so is aimed at children who are very definitely pre-adolescent. This is a very distinct problem with the general trend of children's literature in the past thirty years; there has been a lot of focus on RELUCTANT readers and those with poor foundational skills, which led to a lot of material with low lexile levels and higher age interest levels. My first grader could easily have read Twilight, but she (appropriately, IMO) had no interest in doing so.
She was interested in books written for children, like L. Frank Baum's Wizard of Oz series, or J.M. Barry's Peter Pan. She just didn't know that they existed until I started steering her toward those books.
Next time you are in your child's classroom, take a look at the class library and ask about the range of lexile levels. It might be very enlightening.
