[jsprogrammer]

Arxiv shows that the most recent revision of their document was published on January 20, 2015 (less than three weeks ago).

I suppose it is possible that Nature's version is substantially different, but if that were the case it would cast a huge shadow of doubt over Nature's editorial process, as well as the claims made in the paper itself.

[TTPrograms]

The differences are likely a few small sentence additions, citations and comparisons to other work. I think the results would remain unaltered.

[montecarl]

Peer review and getting an article published takes at least a month if not half a year or longer. The preprint on Arxiv is most likely the final version of the paper they submitted for publication post peer review.