Question about vertex shader implementation

[Isaac Young]

Hi there!
I'm trying to understand Jacobian calculation in vertex shader code but I can't understand the signs on J[0][0] J[0][1],shouldn't be the same? please enlighten me,Thanks

    mat3 J = mat3(
        focal.x / cam.z, 0., -(focal.x * cam.x) / (cam.z * cam.z), 
        0., -focal.y / cam.z, (focal.y * cam.y) / (cam.z * cam.z), 
        0., 0., 0.
    );