Professional :: Large Vertical Gradients Not Working Like Examples

I am trying to create a simple gui component that has a vertical gradient, simple enough right... I create the sprite, and draw the gradient with beginGradientFill and a matrix with a 90 degree radian angle on it; but it stops working if the sprite has a long width, below is an exmaple of what I mean:

package {
import flash.display.GradientType;
import flash.display.Shape;
import flash.display.Sprite;
import flash.geom.Matrix;
[Code] .....

If I change
matrix.createGradientBox(800, 100, Math.PI*0.5, 0, 0);
to
matrix.createGradientBox(800, 100, Math.PI, 0, 0);
It works fine - but horizontally - why is it that applying 90 degrees the fill does not work?