Might have to have someone correct me, haha but how I understand it: The releases are official releases, the download as zip is for downloading the code that is in the branch right now, and the archive file is for archived repositories?
At this point the official ClassicPress plugin directory doesn’t exist, so, plugin (and theme) developers are left to their own devices in terms of how they make their titles available. For example, I was originally serving mine from the big green “Clone or Download” button, but, have since changed – that method adds -master to my directory! Instead, I’ve moved over to creating releases and will create a better interface (on my own site) that allows users to download them directly from GitHub (but, via my site.) Once the official directory is built, we will have clear guidelines and the process will be made consistent.