A question regarding singleton

[LarryChen]

Here is a simple implementation of singleton,

class A
{
private:
	A(int x) : _x(x)
	{
	}
    int _x;

	friend A& createA(int x)
	{
		static A a(x);
		return a;
	}
};

int main()
{
	A a1 = createA(1);
	A a2 = createA(2);

	return 0;
}

By debugging, I found that a1._x = 1 and a2._x = 1. It looks like only one object is created. But I wonder why returning static object a can guarantee only one object is created.