It looks like Ubuntu 16.04 comes with Nginx 1.9.15, which is both not the latest stable release (it's a development, aka MAINLINE, release, although Nginx development branch is pretty stable) and it's one minor version ahead of Nginx's own development PPA, which is at 1.9.14.
No, many people get this wrong. Mainline is stable, and http://hg.nginx.org/nginx/ is dev. The labeled stable version is for distros that have strict upgrade policies.
nginx recommends using mainline over stable: "We recommend that in general you deploy the NGINX mainline branch at all times." [1]
> It looks like Ubuntu 16.04 comes with Nginx 1.9.15, which is both not the latest stable release (it's a development, aka MAINLINE, release, although Nginx development branch is pretty stable)
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.
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