Jagged arrays are faster?

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

public class Test : MonoBehaviour {

    // Use this for initialization
    void Start() {
        StartCoroutine(RunTest());
    }

    private IEnumerator RunTest() {

        yield return new WaitForSeconds(0.5f);

        var size = 10000;
        var stopwatch = new System.Diagnostics.Stopwatch();

        var a = new int[size, size];
        var b = new int[size][];
        for(int i = 0; i < size; i++) {
            b[i] = new int[size];
        }

        stopwatch.Start();
        for(int i = 0; i < size; i++) {
            for(int j = 0; j < size; j++) {
                b[i][j] = 5;
            }
        }
        stopwatch.Stop();

        Debug.Log(stopwatch.ElapsedMilliseconds);
        stopwatch.Reset();

        stopwatch.Start();
        for(int i = 0; i < size; i++) {
            for(int j = 0; j < size; j++) {
                a[i, j] = 5;
            }
        }
        stopwatch.Stop();

        Debug.Log(stopwatch.ElapsedMilliseconds);


    }

}