Years ago I decided that I am of well above average in intelligence, reading comprehension, and mathematical ability. In the vast majority of cases where I find instructions unclear, I don't blame myself.
What I would ask you is, was this a test of reading comprehension or was it a test of subtraction? If the former, then I think we would agree that the question could have been clearer.
If the test was intended to guage the students understanding of subtraction, was it successful? That is, do we properly conclude that the student does not understand that she should use subtraction to find the difference between a quantity of one thing and a quantity of another?
For some reason I found that the two chemistry instructors I had, in high school and college, were both lacking in the ability to properly apply logic and the ability to clearly craft a technical question. The result was that I could understand exactly what was taught in the class, but I couldn't manage to consistently choose the answer that the instructor intended on an examination.
The result was that I was usually successful in challenging the instructors during the review of the exam, but this created a conflict with the instructor which obviously made them unhappy with me because it diminished them in the eyes of the other students.