I think, at that age, it is a question of ability to take on others' perspectives. When being asked a math word problem, if it is reasonably easy to insert yourself or someone you know into the problem, then it is easier to "see" the problem and figure out the solution. However, if a child has to imagine some child who they do not know (an unfamiliar name), then that is an extra step to working the problem and this can make it harder for them to "see" the problem and figure out the solution. Ability to see from others' perspectives improves with age, in general. Maybe this would be more true of visual-spatial learners, though.
