Digitally (fully) sign a Package (eg. docx or xlsx)

As we have seen here: https://msdn.microsoft.com/en-us/library/Aa905326.aspx 

it is reasonably easy to sign a document package. However if I do this with my example .docx document, I always receive a "partly signed" document. In comparison, if I sign the document with Word itself.... next time opened, it shows "fully/completely signed" with signature Xxxx Xxxx. This confuses users and maybe it is even just a very unimportant part, that is signed. 

What went wrong? I want a completely signed document.

Here is my working example:

            using (var wd = WordprocessingDocument.Open(ofd.FileName, true))
            {
                Package wdPackage = wd.Package;
                List<Uri> packageParts2sgn = wdPackage.GetParts().ToList()
                        .Where(part => !PackUriHelper.IsRelationshipPartUri(part.Uri))
                        .Select(p => p.Uri)
                        .ToList();
                {

                    PackageDigitalSignatureManager dsm = new PackageDigitalSignatureManager(wdPackage);
                    PackageRelationshipCollection relationships = wdPackage.GetRelationships();
                    List<PackageRelationshipSelector> relShipSelectors = relationships.Select(
                                rl =>
                                    new PackageRelationshipSelector(
                                        rl.SourceUri,
                                        PackageRelationshipSelectorType.Id,
                                        rl.Id)).ToList();

                    X509Store store = new X509Store(StoreLocation.CurrentUser);
                    store.Open(OpenFlags.ReadOnly);

                    X509Certificate2Collection collection = store.Certificates.Find(X509FindType.FindBySerialNumber,"600d923c00010014444e",true);
                    var certificate = collection[0];
                    dsm.CertificateOption = CertificateEmbeddingOption.InSignaturePart;
                    dsm.Sign(packageParts2sgn, certificate, relShipSelectors);

                }