Hi Alejandro,
I think the source of the confusion is that "Should normally be" and how you should write a default value don't match up because it is normal to use non-default values. So systems are commonly set up with localstatedir=/usr/var and runstatedir=/var/run, despite the fact that their defaults are $prefix/var and the result of appending /run to it. The reason to give the defaults in this form is so that, when someone sets prefix to /usr/local, they (by default) get /usr/local/var and /usr/local/var/run and thus conform to "how your non-system GNU install should work" rather than stomping on where things would have been if you'd hard-coded what many folk do in fact set up. This way, if someone sets things up differently, you play nicely with their choices, rather than hard-coding what "everyone" else does. It's about whoever administers a particular box being at liberty to do what they want, so if you're writing software to offer to others to install on their systems, your defaults should be "polite" and not impose on them any particular set-up. Well written.
