I couldn't find anything specific to MAP testing but here's an explanation from another test that may help:
http://www.p12.nysed.gov/osa/assesspubs/pubsarch/rawscoretoscaledscoreconversionTwing1021.pdf
Quote
Scaled scores help communicate student performance:
� supports the reporting of performance standards across years;
� facilitates equivalency of test forms across the years;
� standardizes the meaning of performance across testing sessions;