请问NMUDP组件中用到的TMemory中流的操作方法？

procedure TForm1.NMUDP1DataReceived(Sender: TComponent;
  NumberBytes: Integer; FromIP: String; Port: Integer);
var
   mStream:TMemoryStream;
   strData:String;
begin
   mStream:=TMemoryStream.Create;
   Try
     NMUDP1.ReadStream(mStream);
     SetLength(strData,NumberBytes);
     mStream.Read(strData[1],NumberBytes);//为什么是StrData[1],而不是StrData呢？
     txtGet.Text:=StrData;
   finally
     mStream.Free;
   end;
end;