C++ double divided by double results in rounded number

I have the following calculation:

    double a = 141150, b = 141270, c = 141410;
    double d = (a + b + c) / 3;
    cout << d << endl;

The output shows d = 141277, whereas d should be 141276.66666666.... The calculation consists of double additions and a double division. Why am I getting a result that is rounded up?? By the way d = (a + b + c) / 3.0 doesn't help.

However in another similar calculation, the result is correct:

    double u = 1, v = 2, x = 3, y = 4;
    double z = (u + v + x + y) / 4;

z results in 2.5 as expected. These two calculations are essentially the same, but why different behaviors?

Lastly, I know C++ automatically truncates numbers casted to lower precision, but I've never heard of automatic rounding. Can someone shed some light?