Created attachment 577 [details]
screenshot

Having a TBarSeries with MarksLocation as mlCenter, the mark is almost in the centre, but if you look it closely you'll see it is a few pixels displaced.

You can see it if you have a bar smaller than the rest, so the bar bounds is similar to the mark bounds:

uses Series;

procedure TForm1.FormCreate(Sender: TObject);
begin
  Chart1.View3D:=false;

  with Chart1.AddSeries(TBarSeries) as TBarSeries do
  begin
    FillSampleValues();
    YValue[2]:=MaxYValue/10;
    MarksOnBar:=true;
    MarksLocation:=mlCenter;
  end;
end;