There is a bit more detail on how the deployments notes are generated here, including how they differ for pre-release versions.
The short answer though is that Octopus is simply combining the releases notes of all versions between the release being deployed and the last successful release.
We are having three channels , with version rules
alpha (4.6)
beta (4.5)
gamma (4.4)
recently we have modified our version rules such that, we accept the below package versions
alpha (4.7)
beta (4.6)
gamma (4.5)
we have created a new release and deployed using beta channel (4.6).
Now when we pull the changes, it is taking releases from from alpha channel as well.
So, when constructing the deployment notes, Octopus will take into account any full version release within the project, regardless of which channel the release was created in.