Help me describe what is going on here

There is something I am confused about with a camera movement tutorial, more specifically with mouse movement (normal windows messaging, not directinput)

Here is the code I am confused about

Code:

---

        // Get the direction the mouse moved in, but bring the number down to a reasonable amount
        angleY = (float)( (middleX - mousePos.x) ) / 500.0f;               
        angleZ = (float)( (middleY - mousePos.y) ) / 500.0f;

---

Shouldn't the zAngle be the horizontal movement (meaning the differnces between the x positions) and the y angle simply be the up/down movement? The zangle is used to find a vector to rotate the up/down movement about. this is weird because when you just examine the y movement the axis rotating about doesn't change! this is from a tutorial, www.gametutorials.com

Thanks if you can help me

it depends on how u use the angley and anglez later right?