Do you use the same dress for all the occasions?
No. Right ? You have different dresses – expensive and cheap, warm and cold etc. with different colours.
But when it comes to studies, children typically apply the same techniques for all the subjects. Some get very good results in some subjects but do bad in a few other subjects.
The very fact that they produce excellent results in some subjects, prove that they are intelligent.
The fault is in the methodology they apply for rest of the subjects in which they score abysmally low. Subjects with Problems (like Mathematics) need an altogether different approach than a theoretical subject like Social studies.