It turns out the reason for this is that those packages are newer in gutsy than in harder, e.g. firefox:
hardy 2.0.0.10+2nobinonly-0ubuntu2 gutsy 2.0.0.6+2nobinonly-0ubuntu1 security 2.0.0.11+2nobinonly-0ubuntu0.7.10 updates 2.0.0.11+2nobinonly-0ubuntu0.7.10
This confuses the upgrader.
It turns out the reason for this is that those packages are newer in gutsy than in harder, e.g. firefox:
hardy 2.0.0.10+ 2nobinonly- 0ubuntu2 2nobinonly- 0ubuntu1 2nobinonly- 0ubuntu0. 7.10 2nobinonly- 0ubuntu0. 7.10
gutsy 2.0.0.6+
security 2.0.0.11+
updates 2.0.0.11+
This confuses the upgrader.