C# Input.GetAxis Increment/Decrement Value

What value is used when you press a key to increase/decrease an Axis? I'm trying to make my own custom input.getAxis script and would like to know. I tried using Time.deltaTime but it seems off when I compare it to the regular input.getaxis.

 using UnityEngine;
 using System.Collections;
 
 public class CustomAxis : MonoBehaviour {
 public string[] inputStrings;
 public float axis;
 
     void Update () {
         if (Input.GetKey(inputStrings[0])){
             if(axis < 1){
                 axis += Time.deltaTime;
             }
         }
         if (Input.GetKey(inputStrings[1])){
             if(axis > -1){
                 axis -= Time.deltaTime;
             }
         }
         if (!Input.anyKey){
             if(axis < 0){
                 axis += Time.deltaTime;
             }
             if(axis > 0){
                 axis -= Time.deltaTime;
             }        
         }
     }
 }