You raised a student’s grades for working hard. Is this fair to others who fared poorly?

As a teacher, after you assigned final grades, two students asked you to reconsider the Cs they got.

As a teacher, after you assigned final grades, two students asked you to reconsider the Cs they got. One got just what his test scores indicated. The other was close to a B-minus and while he did poorly early on, he worked hard and improved significantly. You raised his grade to a B-minus, but not that of the other student. Nowhere do you state that improvement can be a factor in grading. Did you unfairly treat these students differently?