I'm guessing not, since aggregate MAP scores go well above 242, and in any event his ability to do computation will be limited by what he's able to do conceptually. Let's say a hypothetical youngster hadn't been exposed to long division, or division at all as a concept; he wouldn't be able to be tested on his computation ability with division, would he?
I guess that one could "ceiling" in computation by getting none wrong in all conceptually mastered areas, and none right in any other areas (to avoid the testing program assuming that some of the errors were due to computation failures). Maybe that's what happened with your DS-- there were types of questions he got 100% correct with no calculation or other faults, but the rest of the categories of questions were sharply delineated where he got 100% wrong-- but it's impossible for me to draw that conclusion.
I tried looking up "computation" in the documents I have on the NWEA MAP, as I suspect you might've already, but found nothing helpful.
That's a big bump from winter to spring in computation; was he doing lots of drills for the second half of the year? Is that your concern, that he's being asked to spend too much time on drills?