C++ Warming std vector

Why filling std::vector second time is FASTER? Even if space was reserved from the beggining?

int total = 1000000;

struct BaseClass {
  float m[16];
  int id;

  BaseClass(int _id) { id = _id; }
};

int main() {

  std::vector<BaseClass> ar;
  ar.reserve(total);

  {
    auto t_start = std::chrono::high_resolution_clock::now();
    for (int var = 0; var < total; ++var) {
      ar.emplace_back(var);
    }
    auto t_end = std::chrono::high_resolution_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(
                     t_end - t_start).count() << "\n";
    ar.clear();
  }

  {
    auto t_start = std::chrono::high_resolution_clock::now();
    for (int var = 0; var < total; ++var) {
      ar.emplace_back(var);
    }
    auto t_end = std::chrono::high_resolution_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(
                     t_end - t_start).count() << "\n";
    ar.clear();
  }

  {
    auto t_start = std::chrono::high_resolution_clock::now();
    for (int var = 0; var < total; ++var) {
      ar.emplace_back(var);
    }
    auto t_end = std::chrono::high_resolution_clock::now();
    std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(
                     t_end - t_start).count() << "\n";
    ar.clear();
  }

  return 0;
}

online preview: http://coliru.stacked-crooked.com/a/229e4ba47adddb1a

RESULTS:
118
23
21

P.S. I'm asking why it becomes faster if the only reason for slowdown for vector is allocation/reallocation. And we allocated array BEFORE start.