Why when fading out list of materials each material fade out in another time and not all the materials fade at the same time?

Question

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using UnityEngine;

public class FadeInOut : MonoBehaviour
{
    List<Material> mats = new List<Material>();

    private void Start()
    {
        foreach (Transform g in transform.GetComponentsInChildren<Transform>())
        {
            if (g.GetComponent<SkinnedMeshRenderer>() != null)
            {
                var l = g.GetComponent<SkinnedMeshRenderer>().sharedMaterials.ToList();
                foreach(Material mat in l)
                {
                    mats.Add(mat);
                }                
            }
        }  
    }

    private void Update()
    {
        if(Input.GetKeyDown(KeyCode.G))
        {
            StartCoroutine(FadeTo(mats, 0, 0.5f));
        }
    }

    IEnumerator FadeTo(List<Material> materials, float targetOpacity, float duration)
    {
        for (int i = 0; i < materials.Count; i++)
        {
            Color color = materials[i].color;
            float startOpacity = color.a;

            float t = 0;

            while (t < duration)
            {
                t += Time.deltaTime;
                float blend = Mathf.Clamp01(t / duration);

                color.a = Mathf.Lerp(startOpacity, targetOpacity, blend);

                materials[i].color = color;

                yield return null;
            }
        }
    }
}

The first material fade out after 0.5f second but then the other materials each one fade out in a bit more time the last one almost 1-2 seconds after the first one.

and i want all the materials to fade out a the same time if the time is 0.5f or 5f then fade out all the materials after 5 seconds or after 0.5f but the same time and not one after the other.