Why do some but not all held packages block a dist upgrade?

[Adam Funk]

I'm currently upgrading another machine to 14.04 using the
do-release-upgrade command.

I tried it a couple of times & got an error to the effect that the
changes couldn't be calculated, possibly because of old packages or
some other things (I forget what).  Then I remembered that I had
installed the 13.04 version of gnumeric (& related packages) & used
'apt-mark hold' to lock them (because of the freeze-panes bug, unfixed
in the 13.10 version).  I used 'apt-mark unhold' to release them, &
the upgrade is now running fine.

But I still have old versions of faac & libfaac0 on hold, because the
new versions are defective (as far as I'm concerned) because of
licensing issues.  However, these holds are not impeding the upgrade.

Any idea why some holds are OK but some are not?

Thanks,
Adam