Problems with ITextFrame::GetMatrix function

Can anyone explain the values of the matrix returned from the ITextFrame::GetMatrix() function (wrapper for sTextFrame->GetMatrix()).

The a,b, c and d values which represent the rotation (and possibly the shear deformation) are OK,

but I can't relate the tx and ty values to the position of the text object on the artboard.

As a test I tried the following code:

AIErr

CreatePointText( short paintOrder, AIArtHandle prep,

                 AIRealPoint *textPoint, AIArtHandle *newText)

{

     AIError error;

     error = sAITextFrame->NewPointText( paintOrder, prep,

                                         kHorizontalTextOrientation,

                                         textPoint, newText);

     TextFrameRef ateTextRef;

     error = sAITextFrame->GetATETextFrame( *newText, &ateTextRef);

     ATE::ITextFrame ateText( ateTextRef);

     AIRealMatrix matrix = (AIRealMatrix) ateText.GetMatrix();

}

But the tx and ty values returned in matrix are now both 0.  They should equal textPoint.h and textPoint.v, shouldn't they?