What does it mean to have the grade "C"? Does it mean that your work is just average in comparison to the officers in the same grade as yours?

I am asking as I have been given many responsibilities last year (presenting at international conference, participating in partnership research with other institutions, etc) and frankly, I was expecting a better grade - at least a C+ because during the previous years when I was doing less, I was also given a C. This is also not my promotion year - have not had any promotion since 2 years ago.