Andrew M.A. Cater:
/etc/os-release just contains major version
You are going to have to explain that to its manual page, which gives VERSION_ID=11.04 as an example of what can be in the file.
You're going to have to explain it to the Ubuntu people, as well; because they follow what the manual says. (-:
In reality, /etc/os-release can contain both major and minor versions, and does on Ubuntu, OpenSUSE, and so forth. Why it does not on Debian is not something that is imposed by the file format.