Take a look at page six of the transition guidance document for MAP primary vs 2-5, and you will see that, above math RIT 200, the standard error shoots way up.

https://www.nwea.org/content/uploads/2015/01/MPG-to-MAP-Transition-Guidance-Document-JAN15.pdf

Also, note that the survey with goals (adaptive) version of the test stops after about 50 items, so a difference of one or two careless errors can send the score up or down by quite a bit when you are already scoring in the upper extremes. (Hence the high standard error.)

I wouldn't worry too much about the score drop. Suffice it to say he has been and currently is scoring at the meaningful ceiling of the instrument. Measurement of further growth will have to come through some other assessment.

...pronounced like the long vowel and first letter of the alphabet...