Constant motion when applying a constant force?

I have a bullet that shoots quickly. I have made a function to find the distance traveled by the bullet each frame. The results I have got look correct accept for occasional jumps that I cannot explain. Here is an extract of my results:

1.779392, 0.4, 0.8, 1.2, 1.6, 3.6, 2.4, 2.8, 3.2, 3.599999, 7.599998, 4.399999, 4.800001

I am fairly new to coding and very new to Unity so I am not sure if the reason for these jumps is a problem in my code or if Unity's motion is not steady. Can anyone explain?

FYI the code I use to accelerate the bullet is:

 void FixedUpdate ()
 {
     rigidbody.AddForce(transform.up * power, ForceMode.Acceleration);
 }