One thing that can happen is somebody gets a NSERC CGS, but wants to take it up outside canada. In this case they are only allowed a PGS, so they can decline the CGS to accept a PGS. At the doctoral level the difference is thousands of dollars, so if a few people do this, another PGS award opens up.
Also, situations can simply change. They apply thinking they are going to grad school, something comes up in life, and they decide not to.