The offset isn't meant to change the bounds of the callout bubble, but you can still do that with the help of FillSceneGraph event.

The annotation is actually two objects: a callout and a text, so you need to set bounds on both. Like so:

private void ultraChart1_FillSceneGraph(object sender, Infragistics.UltraChart.Shared.Events.FillSceneGraphEventArgs e)
{
 if (e.Grid.Count == 0) return;

 xAxis = (IAdvanceAxis)e.Grid["X"];
 yAxis = (IAdvanceAxis)e.Grid["Y"];

 Callout annotation = null;
 Text annotationText = null;

 foreach (Primitive p in e.SceneGraph)
 {
  if (p is Callout)
  {
   annotation = p as Callout;
   int annotationIndex = e.SceneGraph.IndexOf(annotation)+1;
   annotationText = e.SceneGraph[annotationIndex] as Text;
  }
 }

 if (annotation != null && annotationText != null)
 {
  annotation.Origin = new Point(annotation.Origin.X, (int)yAxis.MapMinimum);

  Rectangle bounds = new Rectangle(
   annotation.BubbleRectangle.X,
   annotation.Origin.Y - 25,
   annotation.BubbleRectangle.Width,
   annotation.BubbleRectangle.Height);

  annotation.BubbleRectangle = bounds;
  annotationText.bounds = bounds;
 }
}