From the description I thought Git-Unstable might be a nightly update. When I look at Git-Unstable the date is 3/17. Developer is is dated 03/12.

When are the releases updated?

User Edition Date 03/21
Featuring the latest officially released KDE software on a stable base.

Git-Unstable 03/17
Featuring pre-release KDE software built the same day from new feature branches.

Developer 03/12
Featuring pre-release KDE software built the same day from bugfix branches.

The isos are normally built daily, but sometimes there may be an issue preventing it, say a problem with the automation, or various components don't build, or failed tests.

Someone on irc or their mailing list may know more, if they don't pop in here.

They used to be daily but are weekly now, still having the if-nothing-goes-wrong caveat though. Daily rotation of all ISOs was too much to bear for the mirror network

Oh, and I'll add that the packages are still built as necessary daily. It's just that the ISOs do not get refreshed all the time, meaning there may or may not be a mountain of updates waiting after installation.