ZXing.Net ITF Barcode issues Windows Phone 8.1

Question

I can successfully decode barcodes ITF using ZXing.Net, but in some cases the Result.Text have some missing numbers, like in this:

original barcode: 836900000008262500403007233338786038100661049195

Result.Text       : 83690000000  26250040300  23333878603  10066104919

In this case there are missing a 8,7,8 and 5

In this other case, the numbers are reordered to a random order and have some missing numbers:

original barcode: 23793381285017475716618000050809162310000010000

Result.Text         23791623100000100003381250174757161800005080

Any ideas why is it happening?

Thanks

EDIT: 06/12/2015

If I don't specify the PossibleFormats the decoder decodes all the codes (the images are here:1drv.ms/1B6wD5c) as ITF and the result is the same as the described above. The code I'm using is here:

    public BarCodeReaderService(IDialogService _dialogService)
    {
        dialogService = _dialogService;

        barcodeReader = new BarcodeReader
        {
            Options = new DecodingOptions
            {
                PureBarcode = true,
                TryHarder = true,
                PossibleFormats = new BarcodeFormat[] { BarcodeFormat.ITF }
            },

        };
    }

    public async Task<string> ScanBitmapAsync(StorageFile file)
    {
        string result = null;
        using (var stream = await file.OpenReadAsync())
        {
            // initialize with 1,1 to get the current size of the image
            var writeableBmp = new WriteableBitmap(1, 1);
            writeableBmp.SetSource(stream);
            // and create it again because otherwise the WB isn't fully initialized and decoding
            // results in a IndexOutOfRange
            writeableBmp = new WriteableBitmap(writeableBmp.PixelWidth, writeableBmp.PixelHeight);
            stream.Seek(0);
            writeableBmp.SetSource(stream);


            try
            {
                result = ScanBitmap(writeableBmp);
            }
            catch (Exception ex)
            {
                dialogService.ShowAsync("Ocorreu um erro \n" + ex.Message, "Erro");
            }
        }

        return result != null ? result : null;
    }

    private string ScanBitmap(WriteableBitmap writeableBmp)
    {

        var result = barcodeReader.Decode(writeableBmp);

        return result != null ? result.Text : null;
    }

Answer 1

The missing numbers are indeed validation numbers and they have to be calculated.