I asked on #debian-mentors, and Paul Wise pointed me to a bug report. The entries you see are caused by a bug. See the Debian bug report http://security-tracker.debian.org/tracker/data/releases broken.
07:05 < pabs> faheem: https://bugs.debian.org/610222
07:06 < pabs> faheem: I guess patches would be welcome
07:15 < ats> faheem: so my first guess was that it was a Plan 9-style code,
but no, it's just a bug
07:16 < ats> I think what's happening is that it's taking a list of
architectures along the lines of "i386,armhf,kfreebsd-i386" and
splitting it by characters rather than on ,
07:16 < ats> so that's actually a list of all the characters that are used in
Debian architecture names ;-)
07:17 < ats> line 407-ish in http://anonscm.debian.org/viewvc/secure-
testing/lib/python/security_db.py?view=markup suggests that it
does split on , first, though, so presumably that's not getting
used properly when fetching the arch field from the database...
Note. Patches welcome. :-)