The ClickedPoint function in TCustomSeries.Clicked funtion, in Series.pas has this condition:
if CheckPointInLine or (FDrawArea and PointInArea(Index)) then
So when the mouse is in the area region, it is considered as if the mouse were over the line segment and the predecessor point is returned (or successor if axis.inverted).
It would be interesting to have the option to always return the nearest point, even when the mouse pointer is over the area region. So we would have:
if CheckPointInLine or (FDrawArea and (not FNewProperty) and PointInArea(Index)) then
FNewProperty could be called FAbsoluteNearest and be false by default, for example.
But other series may be also considered, like TBarSeries. In the TBarSeries, the TCustomBarSeries.Clicked function should check it too, for example like this:
    if not FNewProperty then
    for t:=FirstValueIndex to Min(LastValueIndex,Count-1) do
Forums link:
http://www.teechart.net/support/viewtopic.php?f=3&t=12332&view=unread#unread
Testing code:
uses Series, TeeTools;
var Chart1: TChart;
procedure TForm1.FormCreate(Sender: TObject);
begin
  Chart1:=TChart.Create(Self);
  Chart1.Parent:=Self;
  Chart1.Align:=alClient;
  Chart1.View3D:=false;
//  Chart1.AddSeries(TPointSeries).FillSampleValues(10);
//  Chart1.AddSeries(TLineSeries).FillSampleValues(10);
//  Chart1.AddSeries(TAreaSeries).FillSampleValues(10);
//  Chart1.AddSeries(THorizAreaSeries).FillSampleValues(10);
//  Chart1.AddSeries(TBarSeries).FillSampleValues(10);
//  Chart1.AddSeries(THorizBarSeries).FillSampleValues(10);
//  Chart1.Axes.Left.Inverted:=true;
//  Chart1.Axes.Bottom.Inverted:=true;
  with (Chart1.Tools.Add(TNearestTool) as TNearestTool) do
  begin
    Series:=Chart1[0];
    Pen.Color:=clRed;
  end;
  Chart1.OnMouseMove:=Chart1MouseMove;
end;
procedure TForm1.Chart1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
var tmpIndex: Integer;
begin
  with (Chart1.Tools[0] as TNearestTool) do
  begin
    Caption:='Series clicked: ' + IntToStr(Chart1[0].Clicked(X,Y)) + ',  Nearest point: ' + IntToStr(Point);
  end;
end; [created:2011-05-16T16:38:00.000+01:00 reported by:yeray@steema.com reported in version:2011.03.30407 (TeeChart VCL)]