As Avrom has noted threads are getting accidentally hijacked in the
Google Group.
My understanding is this occurs because at some point somebody takes
one of the original emails, replies, changes the subject line and
sends it back in. Apparently Google Groups doesn't track threads by
the subject line but some other mechanism, and will happily allow the
subject of the thread to change.
The solution is to only post replies in the Google Group itself and
it's something we'd like encourage for the health of the threads.
However that's something we can't enforce regardless of how blue in
the face we get telling people, so not to worry, carry on, one
limitation of using a free service I guess.
Cheers,
CM.