[dudleypippin]

I think the key bit is where "my" is in that second sentence. At the time, the main server (or your hosting service) would, indeed, have the full list of messages for all the groups they carried, with a lookback of a certain number of days (the "retention"). The retention and the groups that a hosting service subscribed to would be part of how you would pick which to sign up with.

Your disk, however, would only have the messages you'd specifically requested and pulled down over your modem. (Or, if you went way back to the vt100 days, "your disk" was the central server which had it all.)