Just to explain this another way, availability for a flight in FC might show (on ExpertFlyer) as J9 C9 D9 I7 Z1, which means they'll sell at least 9 seats in J/C/D fare classes, up to 7 in I and only 1 at a Z fare. Ignoring the dual inventory check mentioned above for simplicity (and I'm not really sure how it factors into the warning), this is what will trigger the "One seat left at this fare". It has nothing really to do with how many seats have already been sold (well, it may be related, but it's more based on how Delta is managing the revenue for the flight), and of course the seat map isn't typically a good indicator of how many seats have been sold, although in domestic first class it usually is fairly reliable. As also mentioned, if you search for 2 passengers, it will only show you the "I" fare, so you can at least see what the price is.

And of course, this can happen with economy fares as well (e.g., Y9 B9 M9 H5 K1, etc.)