Unreliability of Physics.Raycast.Distance for player movement?

I have a script which tries to detect for collisions with Raycasting being executed on the Update() function. However, my tests proved this to be unreliable, since the distance is different every time I run the game and test the Hit Distance of the Raycast.

What I'm trying to do is: cast a Ray to the ground, if a collision is detected and the minimum distance is reached, then stop vertical movement.

I've attached a picture of two different iterations of the same script being run with BIG differences on the results in-game.

As you can see, on one iteration the Raycast distance was detected when the user was still above the plane. However, on the other one, the player's feet was already underground when the distance was detected.

I've thought of doing this on FixedUpdate(), but not only this feels like a cheap workaround but also, according to this official unity video, I should be using this script on Update(), not the other way around: http://unity3d.com/learn/tutorials/modules/beginner/scripting/update-and-fixedupdate

Now, I want a fluid gameplay and a responsive script. I don't want random things happening on my game and want it to work as I tell it to. I'll attach the scripts below and I'd appreciate any input:

PlayerPhysics.cs

 using UnityEngine;
 using System.Collections;
 
 public class PlayerPhysics : MonoBehaviour {
     public LayerMask collisionMask;
 
     public bool grounded;
     public float _movementSpeed = 5;
     public float _gravity = 4;
     private float _skin = .005f;
 
     private float deltaX;
     private float deltaY;
     private float deltaZ;
 
     private Ray ray;
     private RaycastHit hit;
     private BoxCollider bc;
 
     // Use this for initialization
     void Start () {
         bc = GetComponent<BoxCollider>();
     }
 
     public void Move (float h, float v) {
         deltaX = 0;
         deltaY = 0;
         deltaZ = 0;
 
         Vector3 rayOrigin = new Vector3();
         rayOrigin.x = transform.position.x + bc.center.x;
         rayOrigin.y = transform.position.y + bc.center.y - bc.size.y/2;
         rayOrigin.z = transform.position.z + bc.center.z;
         ray = new Ray(rayOrigin, Vector3.down);
         Debug.DrawRay(ray.origin, ray.direction);
 
         if (Physics.Raycast(ray,out hit, collisionMask)) {
             Debug.Log(hit.distance);
             if (hit.distance > _skin) {
                 deltaY -= _gravity;
             } else {
                 grounded = true;
             }
         }
 
         if (!grounded) {
         }
 
         // Moves the target
         transform.Translate(new Vector3(deltaX, deltaY, deltaZ) * Time.deltaTime, Space.World);
     }
 }

PlayerController.cs

 using UnityEngine;
 using System.Collections;
 
 [RequireComponent (typeof (PlayerPhysics))]
 
 public class PlayerController : MonoBehaviour {
     private PlayerPhysics playerPhysics;
 
     // Use this for initialization
     void Start () {
         playerPhysics = GetComponent<PlayerPhysics>();;
     }
     
     // Update is called once per frame
     void Update () {
         float h = Input.GetAxisRaw("Horizontal");
         float v = Input.GetAxisRaw("Vertical");
         
         playerPhysics.Move(h, v);
     }
 }