basically the difference is in install command:
apt-get --no-install-recommends install postgresql-9.6 postgresql-contrib-9.6
without --no-install... option it messes with dependences and rejects installing, at least in my case. Adding option, results in some warning message, but just continue and everything installs fine.
Moreover as pointed out earlier, the binaries are taken from default debian repo, as it seems no binaries are provided for arm arch. by posgresql team. It seems that adding PostreSQL apt repository is therefore not necessary at all (I deleted the pgdt.list file with repo entry, then again
apt-get update...), BUT YOU NEED TO CHECK IT (I'm still beginning beginner in linux world). What I have found, is this info:
Currently, we support
Debian 7 (wheezy), 8 (jessie) and unstable (sid)
Ubuntu 12.04 (precise), 14.04 (trusty), 16.04 (xenial)
Architectures: amd64 (64-bit x86), i386 (32-bit x86), ppc64el (little-endian 64-bit POWER; not on wheezy/precise)
PostgreSQL 9.1, 9.2, 9.3, 9.4, 9.5, 9.6, 10devel
So no arm there...