Wonderful analysis of Natural Scrolling by J. Eddie Smith:
In iOS, we typically find new information at the top of ticker tapes. Any app that makes use of “pull to refresh” behaves this way.
On a desktop, we normally find new information at the bottom. For example you would scroll down to advance a PDF, word processing document, or email.
In both cases, down resulted in new. With natural scrolling, even though it’s physically equivalent to the behavior in iOS, down now results in old.