That's because if the default parameters are there, this is ambiguous:

```cs
repo.Refs.Add(name, "be3563ae3f795b2b4353bcce3a527ad0a4f7f644");

// error CS0121: The call is ambiguous between the following methods or properties: 'LibGit2Sharp.ReferenceCollectionExtensions.Add(LibGit2Sharp.ReferenceCollection, string, string, LibGit2Sharp.Signature, string, bool)' and 'LibGit2Sharp.ReferenceCollectionExtensions.Add(LibGit2Sharp.ReferenceCollection, string, string, bool)'
```