Discrepancy execution times in parallel programming

I made a parallel code in C to verify its execution time. It was treated:

• Threads;
• Mutex;
• False Sharing;
• Synchronization.

When running time of Linux with code execution, it was generally possible to compute at the following time:

Resultado da soma no modo concorrente = 36028797153181696  

real    0m0.340s  
user    0m1.300s  
sys     0m0.000s  

However, every + - 5 of these executions took a dramatically different time:

Resultado da soma no modo concorrente = 36028797153181696  

real    0m1.863s
user    0m5.584s
sys     0m0.000s  

What did you miss? What do the times real , user , sys mean?

The code that calculates the sum of elements from 1 ~ N

#include <stdio.h>
#include <pthread.h>
#define QTD 268435456 //1024//16384 //8192
#define QTD_N 4

unsigned long long int soma = 0;
unsigned long int porcao = QTD/QTD_N;

struct padding{
    unsigned long long int s;//soma parcial;
    unsigned int i,start,end; 
    unsigned int m; // identificador de parcela
    char p[40];
};

pthread_mutex_t mutex_lock;


void *paralelo(void *region_ptr){
    struct padding *soma_t;
    soma_t = region_ptr;
    soma_t->s = 0;    
    soma_t->start = soma_t->m * porcao + 1;
    soma_t->end = (soma_t->m + 1) * porcao; 
    for(soma_t->i = soma_t->start; soma_t->i <= soma_t->end ; soma_t->i++){
        soma_t->s += soma_t->i;
    }

    pthread_mutex_lock(&mutex_lock);
    soma += soma_t->s;
    pthread_mutex_unlock(&mutex_lock);
    pthread_exit(NULL);
}

int main(void){ 
    pthread_t thread[QTD_N];
    struct padding soma_t[QTD_N];
    int i;
    void *status;
    pthread_mutex_init(&mutex_lock,NULL);
    for(i = 0 ; i < QTD_N ; i++){
        soma_t[i].m = i;
        pthread_create(&thread[i], NULL, paralelo, &soma_t[i]);
    }

    for(i = 0 ; i < QTD_N ; i++){
        pthread_join(thread[i],&status);
    }
    pthread_mutex_destroy(&mutex_lock);

    printf("Resultado da soma no modo concorrente = %lli\n",soma);
    return 0;
}