Why the code is working fine without mutex?

I am trying to learn how locks work in multi-threading. When I execute the following code without lock, it worked fine even though the variable sum is declared as a global variable and multiple threads are updating it. Could anyone please explain why here threads are working perfectly fine on a shared variable without locks?

Here is the code:

    #include <pthread.h>
#include <stdio.h>
#include <stdlib.h>

#define NTHREADS      100
#define ARRAYSIZE   1000000
#define ITERATIONS   ARRAYSIZE / NTHREADS

double  sum=0.0, a[ARRAYSIZE];
pthread_mutex_t sum_mutex;


void *do_work(void *tid) 
{
  int i, start, *mytid, end;
  double mysum=0.0;

  /* Initialize my part of the global array and keep local sum */
  mytid = (int *) tid;
  start = (*mytid * ITERATIONS);
  end = start + ITERATIONS;
  printf ("Thread %d doing iterations %d to %d\n",*mytid,start,end-1); 
  for (i=start; i < end ; i++) {
    a[i] = i * 1.0;
    mysum = mysum + a[i];
    }

  /* Lock the mutex and update the global sum, then exit */
  //pthread_mutex_lock (&sum_mutex);  //here I tried not to use locks
  sum = sum + mysum;
  //pthread_mutex_unlock (&sum_mutex);
  pthread_exit(NULL);
}

int main(int argc, char *argv[])
{
  int i, start, tids[NTHREADS];
  pthread_t threads[NTHREADS];
  pthread_attr_t attr;

  /* Pthreads setup: initialize mutex and explicitly create threads in a
     joinable state (for portability).  Pass each thread its loop offset */
  pthread_mutex_init(&sum_mutex, NULL);
  pthread_attr_init(&attr);
  pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);
  for (i=0; i<NTHREADS; i++) {
    tids[i] = i;
    pthread_create(&threads[i], &attr, do_work, (void *) &tids[i]);
    }

  /* Wait for all threads to complete then print global sum */ 
  for (i=0; i<NTHREADS; i++) {
    pthread_join(threads[i], NULL);
  }
  printf ("Done. Sum= %e \n", sum);

  sum=0.0;
  for (i=0;i<ARRAYSIZE;i++){ 
  a[i] = i*1.0;
  sum = sum + a[i]; }
  printf("Check Sum= %e\n",sum);

  /* Clean up and exit */
  pthread_attr_destroy(&attr);
  pthread_mutex_destroy(&sum_mutex);
  pthread_exit (NULL);
}

With and without lock I got the same answer!

Done. Sum= 4.999995e+11 
Check Sum= 4.999995e+11