Math: Differential Equations
Math: Linear Algebra
could be just different units of Math, they were given in different time. Like another reader said, there were totally 7 or 8 courses. Or there may be a different definition on course in your son's school.
Hope you can check with your son and then enlighten the readers on this, otherwise cause misunderstanding.
Pls don't feel offend, as you intend to provide some info to other parents, then pls make sure the info not cause others confused or push their kids try to take so many courses.