race condition when updating package lists
There is probably a race condition situation when updating package lists, which is the primary reason it is tagged as 'when:manual' right now.
Let's say Ubuntu introduces package X with version Y, and it takes a couple of days until that hits Trisquel. If the package updating list (and the debdistget scripts...) runs during that period, it will think that Trisquel introduced a package difference, so it would want to rebuild the Trisquel-version since it is now no longer in Ubuntu. This would happen even if the package in Trisquel came from Ubuntu originally, just that it is an older version.
So for that reason, when running 'generate-package-lists.sh' manually, you have to take care that Trisquel has imported all "intended" packages from Ubuntu, which is a somewhat manual process until we figure out how to deal with this.