ScreenToViewportPoint with MousePosition giving -0.1 instead of 0

Hi!

I was wondering why using Input.MousePosition with ScreenToViewportPoint is giving me -0.1 in x when I move my mouse to the very left of the screen, and -0.1 in y when moving to the very bottom of the screen.

I find it odd as I thought ScreenToViewportPoint only gave mouse coordinates on a 0 to 1 scale.

Why am I getting something lower than 0 and is there a way I can fix this? I have provided my code below in case you notice I'm doing something incorrectly.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class CameraController : MonoBehaviour
 {
     // Constant reference to mouse position as a screen point
     // Check if that reference is on the left, right, top, or down
     //  and maybe diagonal.
     Vector3 mouseScreenPos;
 
     // Use this for initialization
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         mouseScreenPos = GetComponent<Camera>().ScreenToViewportPoint(Input.mousePosition);
     }
 }

(I would have added line breaks if I knew how to... sorry)