What is happening?

int main()
{
    struct Bits
    {
        unsigned a : 1, b : 3, c : 4,       // struct of bit-fields to hold
                 d : 5, e : 4, f : 4,       // the different number of bits
                 g : 4, h : 4, i : 4,
                 j : 5, k : 2;
    };

    union Bitvalue                          // union to hold bits and value
    {
        Bits bits;                          // struct of bit-fields
        size_t value;                       // variable to extract value
    };

        // define a union with the values supplied in the exercise manual
    Bitvalue bitvalue = {{0, 7, 15, 10, 6, 7, 15, 15, 7, 0, 3}};
    bitset<48> allbits(bitvalue.value);

    cout << allbits << '\n'; // should print 000000001100000011111111111011101100101011111110‬
                             // but prints   000001100000011100011111111011101100101011111110
                             // note the extra 000           ^^^ here
                             // otherwise the sequence is correct, it just adds 3 extra 0 bits
}