Why does GPA swing so much like this? 10 pts?

Why is it that when you get one bad grade, your GPA plummets, and when you get a great one, it skyrockets? Am I right in feeling that the more you stray from your "normal" range, the more it surges in either direction? I am about to graduate and am hoping if I get 5 straight As this semester I can raise my 3.17 GPA to a solid 3.3 at least?