I've noticed that apt-get often does not get the latest versions of packages. For instance, I like to use the ntp alternative chrony. apt-get installs version 1.30 from July, 2014 on my jessie system.
If I lookup chrony on packages.debian.org, I can see that different versions are listed for wheezy (1.24), jessie (1.30), stretch (3.0), buster (3.2), and sid (3.2).
Is packages.debian.org where apt-get looks for packages?
Who decides which package version is made available for the various OS releases?
When running jessie, can I force apt-get to install a newer version like 3.0 or 3.2?