Why is /etc higher priority than /run

From systemd.unit(5) it appears that configuration in /run/systemd has lower priority than /etc/systemd.

This seems backwards to me. That means I can't have runtime overrides that overrule system configuration.

In particular, for a usecase that matters to me, this means that `systemctl mask --runtime` doesn't work if the service is configured in /etc/systemd/system.

What is the logic for having this priority order? And is there a way to temporarily mask a unit that is configured in etc just for this boot?