It's not specific to the MAP test but this may help:
http://lexile.com/about-lexile/grade-equivalent/
Quote
Grade equivalent scores are often misinterpreted as being a grade level standard. A grade equivalent of 5.9, for example, does not represent the desired level of achievement for all grade 5 students. It simply represents the norming group's median score, or projected score, for 5th-grade students in their ninth month of schooling.