Those are fare classes. The airline allocates a certain number of seats to each fare class. When the cheapest class is gone, then the price goes up when they move to the next one.
Well its possible that they will release more discounted tickets (like fare class W or X) but it depends on where you are flying from when you are flying etc. Its really a crapshoot.
>>> So, would it be safe to say that'll be my best deal NOW on that flight?
Yes.
(copied from another post)
Except for really popular travel days, it is often better to wait for a fare war (T class if Delta) rather than buy at their everyday low price.