为什么在使用直接打印时字符串会被截断？

MyPrinter := TRawPrint.Create(nil);
try
  MyPrinter.DeviceName := \'EPSON TM-T70 Receipt\';
  MyPrinter.JobName := \'MyJob\';
  if MyPrinter.OpenDevice then
  begin
    MyPrinter.WriteString(\'This is page 1\');
    MyPrinter.NewPage;
    MyPrinter.WriteString(\'This is page 2\');
    MyPrinter.CloseDevice;
  end;
finally
  MyPrinter.Free;
end;

MyPrinter.NewPage

WriteString

Result := False;
if IsOpenDevice then begin
  Result := True;
  if not WritePrinter(hPrinter, PChar(Text), Length(Text), WrittenChars) then begin
    RaiseError(GetLastErrMsg);
    Result := False;
  end;
end;

WrittenChars