I noticed this same phenomenon with one of mine when we were using the Singapore Math Primary Math test books. During the brief period of time when I was experimenting with using the tests, I switched from the open-response to multiple-choice forms mainly because of handwriting/fine-motor issues, but then found that #2 was a really good guesser on mc problems. Of course, it's not really guessing; it's having good number sense, estimation, and problem-solving skills. All excellent things. Unfortunately, that does not substitute for actually being able to generate precise answers when necessary!
It did help me see that we would have to work on computation and problem solving somewhat independently, though.
