this is generally how Linux distro work. For each ‘major’ version of the distro, the distro choses the version of each individual component/package (such as bluez). For Debian ‘jessie’ which is the version of Debian that we currently use in our images, bluez version is 5.23.
You can see all the various bluez version in the various Debian releases here:
If you start picking up newer (or older) version of packages, then you are customizing the distro. It is possible to do, most of the time, but that requires a bit of backporting.
Because Debian has a fairly long release cycle (~2 years) we have recently decided to switch our images from Debian stable (aka jessie) to Debian testing (e.g. stretch right now). The next release, in March, will be based on Debian stretch, and as such will have bluez 5.43 (and generally speaking, most linux packages should be up-to-date).