Ashley, you're right.

In regards to my hypothetical question and answer, that child really should get full credit. My point was that the provision of extra information is often perceived by students and parents as something positive, or even a source of possible extra-credit... but for teachers (when grading) it is often either a distraction, or a source of unnecessary confusion.

This is particularly confounding for kids as these same teachers are often very encouraging of extra-information in classroom discussions, where it serves to keep conversation moving and adds color and interest to the material being covered.