This appears to be part of the phasing of stable release updates that has been in place since 13.04.
Users of Ubuntu 13.04 who install updates with update-manager are automatically participating in this process.
Basically, updates are released to 10% of users. Then an error report database looks for regressions or problems. If everything is ok, 6 hours later another 10% of users receive the updates. The phasing status tracker is automatically updated accordingly, and you can monitor the process there.
One can opt out of the Phased Update process by adding '
Update-Manager::Never-Include-Phased-Updates "True";' to the configuration file "
On my system that file does not exist, so if you choose to opt-out, you may need to create it with
sudo nano /etc/apt/apt.conf and add the needed line.
On Ubuntu releases 20.10 and earlier,
apt-get will always show updates right away. In Ubuntu 21.04, phased updates came to apt-get.
Also, in System Settings >> Software and Updates, go to the Updates tab and make sure When there are security updates and When there are other updates both say Display immediately (But note that these settings do not override the phased updates.)