WebGL wrong scaling of rectangle

I’m working with this tutorial http://www.html5rocks.com/en/tutorials/webgl/webgl_transforms/

and my result is, that the rectangle moves in the direction of it’s scaling values, and also scales at the same time. So if I scale (2.0, 1.0) it moves right and stretches, but I only want to strecht/scale it. This is my vertex shader, it is nearly the same as in the tutorial (declarations omitted):

void main(void) {
vec2 scaledPosition = a_position * u_scale;

vec2 rotatedPosition = vec2(
    scaledPosition.x * u_rotation.y + scaledPosition.y * u_rotation.x,
    scaledPosition.y * u_rotation.y - scaledPosition.x * u_rotation.x);

vec2 pos = rotatedPosition + u_translation; 

vec2 zeroToOne = pos / u_resolution;
vec2 zeroToTwo = zeroToOne * 2.0;
vec2 clipSpace = zeroToTwo - 1.0;

v_texCoord = a_texCoord;

gl_Position = vec4(clipSpace, 0, 1); 
}

What could be the mistake if it is not in the shader? Thanks in advance.