Condition Variable notify_one notify_all - c++

Question

I'm trying to learn the condition variables, and I'm stuck at the following example. I thought that notify_one on consumers should unlock only one waiting consumer. But after starting it repeatedly it seems to me that this isn't the case. I've changed notify_one into notify_all and haven't noticed a change in behavior. After the producer calls notify_one on consumers I can see Get… being written on screen by more then one consumer.
Why is this happening?
#include <iostream> // std::cout
#include <thread> // std::thread
#include <mutex> // std::mutex, std::unique_lock
#include <condition_variable> // std::condition_variable
#include <chrono>
std::mutex mtx;
std::condition_variable produce,consume;
int cargo = 0; // shared value by producers and consumers
void consumer () {
std::unique_lock<std::mutex> lck(mtx);
while (cargo==0) consume.wait(lck);
std::cout << "Get" << cargo << " "<< std::this_thread::get_id() << '\n';
cargo--;
produce.notify_one();
}
void producer (int id) {
std::unique_lock<std::mutex> lck(mtx);
while (cargo!=0) produce.wait(lck);
std::cout << "Push" << id << " "<< std::this_thread::get_id() << '\n';
cargo += id;
consume.notify_one();
}
void c () {
while(1) {
consumer();
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
}
}
void p(int n) {
while(1) {
producer(n);
std::this_thread::sleep_for(std::chrono::milliseconds(1000));
}
}
int main ()
{
std::thread consumers[5],producers[5];
for (int i=0; i<5; ++i) {
consumers[i] = std::thread(c);
producers[i] = std::thread(p,i+1);
}
for (int i=0; i<5; ++i) {
producers[i].join();
consumers[i].join();
}
return 0;
}