为什么在这种情况下其中一条路径没有绑定？

Elements and FirstAttribute bind as I'd expect (if I didn't know it's a method), but Attributes does not, despite being a member of XElement, just like the others. I know about IValueConverter, and I'm using that to get the binding I want on attributes, but I'm curious as to why it works on Elements.Elements and FirstAttribute bind as I'd expect