Comment 51 for bug 1690980

@Julian: When running u-u at shutdown the expectation is that it runs to completion including the downloads (started by u-u-s).

When running in periodic mode triggered by apt's timer it finishes the transaction on TERM signal then it exits to let the shutdown continue. In this case you are right it does not perform the download and installation steps in the same service. In apt-daily-upgrade's case the 900 second is probably future-proof unless the machine/disk is very slow.

There are still installer packages which need downloads even during installation, but they are not updated very often.