Not sure if here it's the right place to ask, if not – please point me to the right direction.
Let's say there's a package, for the sake of real-world example – bind9. In Precise and in Quantal it's version 9.8.1. The original developer (ISC) currently provide versions 9.8.4 which is a bugfix release in the 9.8 line, and 9.9.2 which is a "new features" branch. It looks like when a security issue is encountered, the specific bugfix is backported into 9.8.1.
Now the question: Why maintainers don't just update to the latest bugfix release ? Why to backport only certain patches ? Is it intentionally or just there's no maintaner who would take the effort to update to the latest bugfix release ?