Because `-additionalContentForURL:ofType:forSaveOperation:error:` is a method specifically for subclasses to override. The content is part of the transaction that is saving, rather than being a property of the document. Unfortunately `NSDocument` has no such concept, so an ivar is the only way to approximate it.

I prefer the raw ivar access for something like this. An internal @property still kinda suggests that other bits of the class are free to adjust it if they see fit. I'd rather keep the underscore as a big warning that callers damn well ought to know what they're doing!