Here's the official explanation (disclaimer?) regarding using NUMATS test scores.
2011 NUMATS Guide to Understanding EXPLORE® Results
EXPLORE test scores can provide direction for educators in considering the need for curricular modification for an individual or for groups of students. Test data from above-level testing provide more specific information than data from grade-level tests because the ceiling effect has been minimized. Standardized test data are informative when considering curricular modifications in a school setting; however, results from one test cannot be considered a comprehensive assessment of a child’s aptitude. There are other factors that need to be considered when making a decision regarding curriculum modification. These factors include motivation, maturity, availability of resources, etc. Often, further evaluation is needed to determine specific modifications.
It's good to remember that NUMATS test scores are only one piece of the puzzle, but an important piece backed up by years and years of research.