The likely reason is the lack of a review.
Being efficient is different from being fast. A person who takes a test very quickly can get a good grade, however, some care should be taken regardless of how long a person can perform a task.
For a student in test, it is important at the end, to review each response given in order to perceive any mistakes or forgetfulness of response markup. If David realized that he responded incorrectly, even knowing the correct answer, what happened was haste, lack of attention and review.
