I, too, have noticed a $5 fare difference between legs for several flights recently. A search of Shuttle flights for several random dates reveals no fare difference between the two legs.
However:
Mar 18-19
LGA-PHL-LGA
LGA to PHL - Fare Basis KXRVNA2 $146.05 (US3625)
PHL to LGA - Fare Basis KXRVNA2 $141.40 (US3716)
PHL-LGA-PHL
PHL to LGA - Fare Basis KXRVNA2 $146.05 (US3716)
LGA to PHL - Fare Basis KXRVNA2 $141.40 (US3625)
For Mar 19-20:
LGA-PHL-LGA
LGA to PHL - Fare Basis KXRVNA2 $146.05 (US3625)
PHL to LGA - Fare Basis KXRVNA2 $141.40 (US3716)
PHL-LGA-PHL
PHL to LGA - Fare Basis KXRVNA2 $146.05 (US3716)
LGA to PHL - Fare Basis KXRVNA2 $141.40 (US2409)
Comparing the color coded legs, which are exactly the same dates, same flights, same fare basis, there is obviously a $4.65 additional charge being added to the first leg in each rt itinerary.
<redacted suggestion to contact US re: false advertising>
<edited to add> Found it - in the fare rules (Expedia <edited - not Travelocity>) for the first itinerary:
12.SURCHARGES
FARE RULE
FUEL SURCHARGE OF USD 4.65 PER FARE COMPONENT WILL BE
ADDED TO THE APPLICABLE FARE.
NOTE - TEXT BELOW NOT VALIDATED FOR AUTOPRICING.
THE FUEL SURCHARGE IS ASSESSED ON EACH FARE
COMPONENT.
GENERAL RULE - APPLY UNLESS OTHERWISE SPECIFIED
AGENCY HAS NO RIGHT TO VIEW.
I'm having trouble figuring out how Expedia <edited - not Travelocity> is coming out with the same $329.80 a/i price as usairways.com based on the data I have (and I'm too tired to look for more), but I think we need more information before jumping to conclusions...