I can't understand wired std::atomic_short.load() behavior

I check C++11 std::atomic_short behavior.
I set either 0 or 255 value to atomic_short variable.
But load() say value is neither 0 or 255.
I want atomic variable that one thread write and another thread read.

Environment:
Intel Core i5
OSX 10.11.6
clang (Xcode7.3.1)

#include <iostream>
#include <atomic>
#include <thread>

std::atomic_short value = ATOMIC_VAR_INIT(0);

void process1() {
    bool flag = false;
    for (int i = 0; i < 100000; ++i){
        std::this_thread::yield;
        if (flag){
            value.store(255);
        } else {
            value.store(0);
        }
        flag = !flag;
    }
}

void process2() {
    for (int i = 0; i < 100000; ++i){
        std::this_thread::yield;
        if (value.load() != 255 && value.load() != 0){
            printf("warningA! %d\n", i);
        }
    }
}

int main(int argc, char** argv) {
    value.store(0);
    std::thread t1(process1);
    std::thread t2(process2);
    t1.join();
    t2.join();

    return 0;
}

---

warningA! 3
warningA! 1084
warningA! 1093