The HorzAlignments is used for the text, not for the bitmap, the bitmap is always left-aligned unless you implement the OnGetCellProperties and change the alignment:
procedure TForm1.TMSFMXGrid1GetCellProperties(Sender: TObject; ACol,
ARow: Integer; Cell: TFmxObject);
begin
if Cell is TTMSFMXBitmapGridCell then
(Cell as TTMSFMXBitmapGridCell).Bitmap.Align := TAlignLayout.Client;
end;