The ppa[1] notes there's a newer version[2] also
[1] https://launchpad.net/~nginx/+archive/ubuntu/development [2] https://launchpad.net/ubuntu/+source/nginx/1.9.15-0ubuntu1
nginx recommends using mainline over stable: "We recommend that in general you deploy the NGINX mainline branch at all times." [1]
The stable branch will fork from the mainline branch shortly. The version shipped in 16.04 is very close to what the stable will be, because the fork hadn't taken place before 16.04's release. I expect there to be very few changes, which is why (as someone else pointed out) we expect to update 16.04 to the stable branch as soon as it is available.
> The ppa[1] notes there's a newer version[2] also
That's just noting that the version released in 16.04 is newer than the version provided in the PPA.
> Nginx's own development PPA
Actually it's a PPA maintained by a team that care about Nginx's availability in Ubuntu. In this case, the uploads to that PPA were made by the very same person who looks after the official Ubuntu Nginx packages available to Ubuntu users by default.