The pricing is also very interesting. Rumors are around $10 million for all the games (http://recode.net/2016/04/05/twitter-beats-amazon-verizon-fo...). The other bids were definitely higher, but I think this was a smart decision by the NFL. If Twitter can pull the live stream off with their strong live social capability, it would be great for both parties.
Disclaimer: The IF that twitter can pull of the live stream is a significant IF, not to be underestimated.