It has to do with whether or not the person being tested is right brain dominant or left brain dominant. The vast majority of people are left brain dominant and for them multitasking is extremely difficult because of the linear sequential nature of the left hemisphere.

Right brained people, those with the right brain dominant over the left, have an easier time multitasking because the right brain is image oriented. The right brain processes information as images are processed, In a visual simultaneous manner.