Each date #1-#8 have a "slot" in the member data. When you add and remove a date field you are accessing one of these pre-defined member fields. So if you enable a field that had a date in the past, that date will re-appear. This is somewhat by design, so if you remove a date field all your date data doesn't get removed, in case you want to re-add it later.
Does that make sense?
Mike