gcc optimization flag -O3 makes code slower than -O2

I find this topic Why is it faster to process a sorted array than an unsorted array? . And try to run this code. And I find strange behavior. If I compile this code with -O3 optimization flag it takes 2.98605 sec to run. If I compile with -O2 it takes 1.98093 sec. I try to run this code several times(5 or 6) on the same machine in the same environment, I close all other software(chrome, skype etc).

gcc --versiongcc (Ubuntu 4.9.2-0ubuntu1~14.04) 4.9.2Copyright (C) 2014 Free Software Foundation, Inc.This is free software; see the source for copying conditions.  There is NOwarranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

So please can you explain to me why this happens? I read gcc manual and I see that -O3 includes -O2. Thank you for help.

P.S. add code

#include <algorithm>#include <ctime>#include <iostream>int main(){    // Generate data    const unsigned arraySize = 32768;    int data[arraySize];    for (unsigned c = 0; c < arraySize; ++c)        data[c] = std::rand() % 256;    // !!! With this, the next loop runs faster    std::sort(data, data + arraySize);    // Test    clock_t start = clock();    long long sum = 0;    for (unsigned i = 0; i < 100000; ++i)    {        // Primary loop        for (unsigned c = 0; c < arraySize; ++c)        {            if (data[c] >= 128)                sum += data[c];        }    }    double elapsedTime = static_cast<double>(clock() - start) / CLOCKS_PER_SEC;    std::cout << elapsedTime << std::endl;    std::cout << "sum = "<< sum << std::endl;}