Could be that preferred/choice seats were left which are not bookable through travel agents or website without a fee. People also book tickets and are not assigned seats so US will open up the seat map and assign seats to those without them.

Keep in mind that seatcounter or the US seat map is not an accurate indicator of what is available on a plane. Special seats-bulkhead and first row of F do not open up until day of departure.

My favorite seat availability tool is the AMEX tool-gives you the actual seats instead of the mystery maps of seatcounter and the US seat map. Here is the link: http://flyaow.com/classamex.htm

On another note, I do not see flight 255 flying PHLPHX tomorrow. You have your flight number right?I see 251, 252, 257 but not 255....Anyway, all flights to PHX show plenty of availability for tomorrow.

Edited to add: I am an idiot-it would help if i searched by the right date. all PHLPHX flights are sold out except the 555pm.