Big reveal here is that Goolge stores a copy of each message everytime it is auto-saved while composing, and never deletes it.
From the Gmail help site: "Please be aware, residual copies of deleted messages and accounts may take up to 60 days to be deleted from our active servers and may remain in our backup systems for an additional period of time." [1]
I assume that previous versions (auto-saved) of emails are treated as deleted emails and are removed after 60 days, but I can't find any documentation for this.