Weird Z position behavior

I'm new to Unity. I have a ball on the ground. The ball has a Rigidbody. In the update function I put GetComponent().velocity = new Vector3(Input.GetAxisRaw("Horizontal"), 0, Input.GetAxisRaw("Vertical")); and after I hit play, before pressing any buttons, the Z position in the inspector keeps changing, to numbers ending with e-some number. Why?