Because IB is a program - there are minimum requirements that must be met. For instance, all diploma candidates must demonstrate a level of proficiency in each of the 6 subject areas. However, this does not preclude someone from specializing in a subject area. Let's look at the example of Art:
A pre-IB student committed to Art would take art electives in grades 9 and 10. The student would then pursue IB Visual Arts courses in grade 11 and grade 12. In addition, the student's Extended Essay could be in the field of Visual Arts. In addition, many of the 150 CAS hours could be 'art allocated' (museums; art galleries; art fundraisers; public relations for arts charities; etc.) The student could also take an additional arts elective in senior year.
4 courses in the Arts; a 4,000 work piece of original research in the Arts (Extended Essay), with Arts' mentors; CAS internships and externships (grades 11 - 12) in the Arts field; This would be an impressive high-school concentration in the Arts.
A similar 'curriculum profile' could be outlined for students specifically skilled or interested in Literature, Math, Social Science, Experimental Science, Foreign Language, etc.