It's because the state is recalibrating its standardized reading tests to make them harder. Why? Because while 87 percent of Tennessee fourth-graders scored "proficient" on state reading tests in 2005, only 27 percent scored "proficient" on national tests the same year.
That data is from a U.S. Chamber of Commerce study released earlier this year (in partnership with the Center for American Progress). Tennessee's toward the bottom of the heap, but it's far from the only state whose metrics are out of whack.