How can parentheses or braces in C++ change variable initial value?

Just an instructive interview question: "What does this C++14 program print?"

#include <iostream>

struct A { int i; };
struct B { 
    A a{1}; 
    operator int() { return a.i; }
};

int main() { 
    B x({});
    B y{{}};
    std::cout << x << y;
}

And it actually prints 10, meaning that the braces in the definition of y change its initial value. But how?