This is due to a change for 7.0 that removed some obsolete update logic for older PMDs. A warning has been added for the case where a domain is opened that is so old that it was not run through all of the upgrade logic that it should have. (This was initially for domains older than 4.2)
Some PMDs are only updated when opened and saved, so any domain that exists in the database that was never opened will stay at the old version. Since we have shipped canned domains by default for many years, some customers have very old PMDs that aren’t updated.