use BitmapData.draw() with BlendMode.ADD cause some borderline？

Hi,every body

I'm trying to draw some circles in a bitmapdata with the BitmapData.draw() method.

when use the BlendMode.NORMAL everything is OK

but when I try with the BlendMode.ADD some borderlines appear.

the main code in following lines:

function drawOneCircle():void

{

    _matrix.identity();

    _matrix.rotate( circle.rotation );

    _matrix.scale( circle.scale, circle.scale );

    _matrix.translate( circle.x, circle.y );

    _colorTransform.color = circle.color;

    _colorTransform.alphaMultiplier = circle.alpha;           

    _bitmapData.draw( bdParticle, _matrix, _colorTransform, BlendMode.ADD, null, true );

}

can anyone help?