Comma initialization and Constructors C++ and Eigen

I'm using Eigen library in C++ and I'm trying to find the determinant of a matrix. I'm getting different results depending on how I initialize the matrices.

Method I:

MatrixXd a(3, 3);
for (int n = 0; n < 3; n++)
    for (int m = 0; m < 3; m++)
        a(n,m) = (double) (n + m*m + 2.5)/3;

cout << "Matrix a: " << endl;
cout << a << endl;
cout << "Determinat of matrix a is: " << a.determinant() << endl;

This part of the code prints

Matrix a:
0.8333333  1.166667  2.166667
1.166667       1.5       2.5
1.5  1.833333  2.833333
Determinat of matrix a is: -7.401487e-17

Method II:

MatrixXd b(3, 3);
b << 0.8333333, 1.166667, 2.166667,
    1.166667, 1.5, 2.5,
    1.5, 1.833333, 2.833333;

cout << b;
cout << endl << "Determinant of matrix b is: " << b.determinant();

which prints

0.8333333  1.166667  2.166667
1.166667       1.5       2.5
1.5  1.833333  2.833333
Determinant of matrix b is: 2.333331e-07

Method I produces the wrong result while Method II gives the right answer. What's going wrong in the first case? (I'm using Visual Studio.) Thanks in advance!