C++ async + future (deferred vs async)

This is the test program that I am working with. Can someone please describe in detail what is happening and the reason for such and output.

Why does launch::async get the g_num value as 0, while launch::deferred gets 100.

Both launch::async and launch::differed got correct values of arg that is on the main stack, which I believe means they should have both gotten 100.

#include <iostream>
#include <future>
using namespace std;

thread_local int g_num;

int read_it(int x) {
    return g_num + x;
}
int main()
{
    g_num = 100;
    int arg;
    arg = 1;
    future<int> fut = async(launch::deferred, read_it, arg);
    arg = 2;
    future<int> fut2 = async(launch::async, read_it, arg);
    cout << "Defer: " << fut.get() << endl;
    cout << "Async: " << fut2.get() << endl;
    return 0;
}

The output:

Defer: 101
Async: 2