Grades are meant to reflect comprehension though. If you get two good grades by killing a bird with one rock, I'd say you understand the material and thus deserve the grades. If the topics don't overlap enough, or if the grades aren't ideal, it means additional effort and/or research should have been applied.
Grades are a means to evaluating things like knowledge and proficiency.
Like you implied, school isn't "real world", and grades aren't either. Interpreting them as their own end, changes the moral equation.