Getting pixel colour from bitmap returning wrong coordinate

Question

I am very new to multi device programming in XE10 Firemonkey and wonder if someone can help me with a problem reading the pixel colour from a bitmap.

What I want to do is sample a colour from a bitmap in a Timage control (like the way an eyedropper colour sampler works). I am showing the colour under the cursor in a TRectangle control.

This is my code in an onmousemove event over the image

procedure TMountboardForm.imagemountboardMouseMove(Sender: TObject;
  Shift: TShiftState; X, Y: Single);
var
  vBitMapData : TBitmapData;
  ASelectionColor : TAlphaColor;
begin
if imagemountboard.bitmap.Map (TMapAccess.maread, vBitMapData) then
begin
  try
   ASelectionColor := vBitmapData.GetPixel (round(X), round(Y)); 
  finally
    imagemountboard.bitmap.Unmap(vBitMapData);
  end;
  rectangle1.Fill.Color :=  ASelectionColor;
end;
end;

This 'sort-of' works but the X & Y coordinates appear to be wrong. It picks up a colour some way away from the cursor rather than the colour under the cursor itself

Can someone tell me where I am going wrong?