The ISO builds are probably on different schedules, and any packaging issues from the “wild” code in Unstable could prevent a new one from being built sometimes. But the ISO date has little to do with what is on it, or rather what packages it will update to, since there will be more current stuff in the actual repos.
The naming is not at all akin to Debian’s here.
User has the most current officially released versions of KDE software.
Testing has alpha-ish to beta pre-release and release-candidate code for KDE and Plasma pulled from git branches that will become the next official releases. Final iterations of new changes and the like to be tested and bug-fixed before the next official release.
Unstable has pre-alpha to alpha quality code pulled from git branches that are much more raw ands rough. New features added and tried out.
So, unless you are looking specifically for the raw and less tested pre-release and unreleased code, User Edition has the most current official versions.