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Because this is not a common core link, but rather a lexile link, it is possible that lexile is trying to remain relevant in the new era of common core?

It's somewhat unclear, but it looks like they drew the numbers semi-directly from CC materials. (Numbers are asterisked and footnoted as coming from *COMMON CORE STATE STANDARDS FOR ENGLISH, LANGUAGE ARTS, APPENDIX A (ADDITIONAL INFORMATION), NGA AND CCSSO, 2012")