Comment 7 for bug 1016294

Archive skew might be to blame here -- a newer version of libnspr4 is built, and gets published on amd64 before i386 (or vice-versa); since the exact same version is required for multiarch installs, apt refuses to install the out-of-sync version.

Try waiting a day or two and just doing it again and see if it changes.

This also might explain why it passes Steve's installability-in-archive test but isn't installable on user systems.