The core of the CCSSO response is:
[T]here is a foundational difference between the purpose of NAEP and the purpose of state exams. There is common agreement that the bar for NAEP proficiency is set at an ideal level for students to achieve, whereas state exams focus on where students need to be for successful lives beyond their K–12 public education. The National Assessment Governing Board, the body that determines the level of NAEP proficiency, clearly states, “Nor is performance at the Proficient level synonymous with ‘proficiency’ in the subject. That is, students who may be considered proficient in a subject, given the common usage of the term, might not satisfy the requirements for performance at the NAEP achievement level."... The Council also believes a more accurate comparison of scores exists between NAEP basic and state proficiency standards as opposed to NAEP proficiency and state proficiency standards, as the NCES report compares.
As the figure below from the NCES study demonstrates, in some states - say Massachusetts, "proficiency" in fourth grade math means proficiency as defined by NAEP. Other states - say Tennessee, define "proficiency" below NAEP's "basic."
Beyond deliberate obfuscation to avoid dealing with some pretty serious problems of this generation's expectations, resource allocation and system performance on behalf of the next, what possible rational purpose can this confusion serve? It can't be serving the interests of students. Students in Tennessee are in fact competing with students in Massachusetts for their own bright futures, to say nothing of students in China. It's one thing to leave states free to decide how they will get their students to world-competitive standards; it's quite another to let them race to the bottom via the defintion of proficiency. In the end, today's truly proficient students in Massachusetts will end up subsidizing their classmates in Tennessee who were merely allowed to believe they were on par. At some point, national standards are inevitable.