Also, bear in mind that unless you're on a purely nonstop pairing, the fare is influenced by the availabilty on every segment. So if you're flying DCA-MIA and the flight is empty, you might get a low fare bucket on the DCA-MIA leg. if your return is MIA-CLT-DCA and MIA-CLT is empty (which would normally avail you to lower fare buckets), but CLT-DCA is a full flight, the lack of low availability on the CLT-DCA leg is going to result in a high fare because the most restrictive fare bucket will apply to the entire return portion.

Finally, last minute fares are often not great deals. A fare booked 45 days advance might be $200 round trip; a 21-day fare may be $300; a 14-day fare may be $375; a 7-day fare may be $450; and a 3 day (effectively walkup) might be $550. That "last minute" fare is almost never as low as the $200 fare. Probably closer to something between the 21- and 14- day fare.