When a new “major” kernel version is released, with a .0 version number (or a short version number, e.g. 4.3), it starts a stable branch which gets updates as necessary. Updates first go to the main branch (Linus Torvalds’ tree), and updates which are relevant for a stable release are back-ported to the various stable branches. Every so often, stable releases are made, and this goes on in parallel on several branches which explains what you’re seeing: 4.4 was released early in 2016 but has continued to receive updates and new releases (4.4.1, 4.4.2...) even as newer kernels were released on more recent branches (4.5, 4.6...) and got updates themselves (4.5.1, 4.6.1...).
On top of that, some stable branches are selected as longterm release branches, maintained for years. This kernel.org pages lists them along with their planned end-of-life. 4.4 is a longterm branch, supported at least until February 2018. 4.8 isn’t, and 4.8.17 was its last release (on January 9).