Timing performance of multithreading - c++

Question

I wrote a small program to check on the performance of threading which I found a couple of questions from the result i obtained
(cpu of my laptop is i5 3220M)
1) The time required pumped up for 2 thread every time I ran the program. Is it because the omp timer I use or I have some logical error in the program?
2) Also will it be better if I use cpu cycle to measure the performance instead?
3) The time continue to decrease as the number of thread increase. I know my program is simple enough so probably requires no context switch but where does the extra performance come? Coz the cpu adj itself to the turbo freq? (Normal 2.6MHz, turbo 3.3MHz according to intel website)
Thanks!
Output
Adding 1 for 1000 million times
Average Time Elapsed for 1 threads = 3.11565(Check = 5000000000)
Average Time Elapsed for 2 threads = 4.54309(Check = 5000000000)
Average Time Elapsed for 4 threads = 2.19321(Check = 5000000000)
Average Time Elapsed for 8 threads = 2.48927(Check = 5000000000)
Average Time Elapsed for 16 threads = 1.84427(Check = 5000000000)
Average Time Elapsed for 32 threads = 1.30958(Check = 5000000000)
Average Time Elapsed for 64 threads = 1.08472(Check = 5000000000)
Average Time Elapsed for 128 threads = 0.996898(Check = 5000000000)
Average Time Elapsed for 256 threads = 1.01366(Check = 5000000000)
Average Time Elapsed for 512 threads = 0.951436(Check = 5000000000)
Average Time Elapsed for 1024 threads = 0.973331(Check = 4999997440)
Program
#include <iostream>
#include <thread>
#include <algorithm> // for_each
#include <vector>
#include <omp.h> // omp_get_wtime
class Adder{
public:
long sum;
Adder(){};
void operator()(long endVal_i){
sum = 0;
for (long i = 1; i<= endVal_i; i++)
sum++;
};
};
int main()
{
long totalCount = 1000000000;
int maxThread = 1025;
int numSample = 5;
std::vector<std::thread> threads;
Adder adderArray[maxThread];
std::cout << "Adding 1 for " << totalCount/1000000 << " million times\n\n";
for (int numThread = 1; numThread <=maxThread; numThread=numThread*2){
double avgTime=0;
long check = 0;
for (int i = 1; i<=numSample; i++){
double startTime = omp_get_wtime();
long loop = totalCount/numThread;
for (int i = 0; i<numThread;i++)
threads.push_back(std::thread(std::ref(adderArray[i]), loop));
std::for_each(threads.begin(), threads.end(),std::mem_fn(&std::thread::join));
double endTime = omp_get_wtime();
for (int i = 0; i<numThread;i++)
check += adderArray[i].sum;
threads.erase(threads.begin(), threads.end());
avgTime += endTime - startTime;
}
std::cout << "Average Time Elapsed for " << numThread<< " threads = " << avgTime/numSample << "(Check = "<<check<<")\n";
}
}