I was able to find PHL-LAX on over 25 departure days in December and January for $178+ roundtrip. We send out notification when an airline drops THEIR cheapest price below all other airlines — and we do it in real time hours before it posts on the airlines website.
 
The airlines decide how many seats and what days they will release THEIR “cheapest” price.
 
In this case Northwest put out this cheap airfare, but they have blackouts on that airfare as noted in the calendar display.
 
The days around Christmas and the days that everyone likes to travel Mon, Friday and Sunday they are charging a premium (at the moment, they could change their minds at any moment which is why most people are frustrated when buying) and are only quoting the 2nd, 3rd and 4th cheapest price.
 
I understand shopping for airfare can be frustrating and we will continue to work on the technology to let you know earlier that the airlines are not selling THEIR cheapest price on the day you want to go — We work on these issues every day (it is our only business) — and will continue to make sure everyone that uses our service makes the best air travel buying decision.
 

There are no 3rd party sites that show real time Southwest price quotes — we would love to have them. Their policy is to only show them on their website right now (and quote them via their phone center).