Depends on how you implement it. The algoritm archieve that gradual movement, just visualize it: Let's suppose that the X value of a projectile which ID is 7 is 2 (X=2). On the first frame, X is increased by using the algoritm, now X is 3. Later on the next frame, the same algoritm is used, but since X is no longer 2 but 3, it will increase to 4, and so on...