Strange result in multithreading using Delphi XE - multithreading

Question

I am trying to use multithreading in Delphi XE.
The task is following I have to create 4 threads. Each thread draw colored circle in Paintbox at predefined area, For example FIRST thread draw only red circles in first quoter of the Paintbox, the SECOND thread draw only yellow circles in the second quoter, and so on.
I have defined following class
const
NumberOfIterations = 100000;
NumberOfTreads = 4;
TCalcThread = class(TThread)
private
FIdx: Integer;
FHits: Cardinal;
V: array of Integer;
xPaintBox1: TPaintBox;
protected
procedure Execute; override;
public
constructor Create(Idx: Integer; vPaintBox: TPaintBox);
property Hits: Cardinal read FHits;
end;
In main code I do the following:
procedure TForm11.Button1Click(Sender: TObject);
var
thrarr: array[0..NumberOfTreads - 1] of TCalcThread;
hndarr: array[0..NumberOfTreads - 1] of THandle;
i, a, t: Integer;
x, y: Integer;
begin
caption := '';
PaintBox1.Canvas.Brush.Color := clWhite;
PaintBox1.Canvas.fillrect(PaintBox1.Canvas.ClipRect);
for i := 0 to NumberOfTreads - 1 do
begin
thrarr[i] := TCalcThread.Create(i, PaintBox1);
hndarr[i] := thrarr[i].Handle;
end;
WaitForMultipleObjects(NumberOfTreads, #hndarr, True, INFINITE);
for i := 0 to NumberOfTreads - 1 do
thrarr[i].Free;
end;
The thread Create and Execute methods are defined as following:
constructor TCalcThread.Create(Idx: Integer; vPaintBox: TPaintBox);
begin
FIdx := Idx;
FHits := 0;
xPaintBox1 := vPaintBox;
case FIdx of
0: xPaintBox1.Canvas.Pen.Color := clRed;
1: xPaintBox1.Canvas.Pen.Color := clYellow;
2: xPaintBox1.Canvas.Pen.Color := clBlue;
3: xPaintBox1.Canvas.Pen.Color := clMoneyGreen;
end;
xPaintBox1.Canvas.Brush.Color := xPaintBox1.Canvas.Pen.Color;
inherited Create(False);
end;
procedure TCalcThread.Execute;
var
i, start, finish: Integer;
x, y: Integer;
begin
start := (NumberOfIterations div NumberOfTreads) * FIdx;
finish := start + (NumberOfIterations div NumberOfTreads) - 1;
for i := start to finish do
begin
case FIdx of
0: begin
x := Random(200) + 1;
end;
1: begin
x := Random(200) + 201;
end;
2: begin
x := Random(200) + 401;
end;
3: begin
x := Random(200) + 601;
end;
end;
y := Random((xPaintBox1.height )) + 1;
xPaintBox1.Canvas.Ellipse(X - 5, Y - 5, X + 5, Y + 5);
end;
end;
As a result I am getting a few circles in three areas with the same color, and a lot of circles in one area (the same color). What I am doing wrong?