Thanks for the responses. I originally searched and mistyped ZFW. Then I fixed my posts with ZFV.

It's strange that on UA's website, if I enter PHL and Search Nearby Airports, ZFV results won't appear even when it's cheaper. Kayak is the site that alerted me of the lower fares, as kayak's search associates ZFV with PHL.

On a side note, It appeared that Continental had ZFV in place, mainly targeted for international flights. e.g. ZFV-EWR-LHR for example. When I used to try ZFV for domestic, it would price very high.

Has UA's strategy with ZFV changed? Because, now it's cheap for domestic flights. I was looking at a ticket on 9/29 where it's $133 CLE-EWR-ZFV one-way. Amtrak is selling the train seat for $59.