MX: Trig question, atan & atan2

I thought that Math.atan2(y, x) computes the arc tan of y/x in radians, and that Math.atan(x) computes the arctan of x in radians. How come then are these two following lines different?

[AS]Math.atan((y1-y2) / (x1-x2));[/AS] and…
[AS]Math.atan2((y1-y2) , (x1-x2));[/AS]
They look to me like they should be the same, but they give very different results.

[fixed typo]