Rates may vary for shipments booked in opposite directions for several reasons. For example, planes going to island locations like Hawaii or to New Zealand are fuller than returning planes because both have to import many of the goods they consume while they export relatively fewer goods. That means the demand for plane space on the way there is higher, so shipping costs on the way there are higher than on the way back.
Even within the U.S., rates can vary by direction due differences in fuel costs or to fees like the remote delivery charge. Such a fee may be assessed in just one direction if only one of the ship-to locations is considered a remote delivery location. For example, you might ship your carbon frame back for repair by dropping it off at a carrier location or bike shop in town, but on its way back to you, the frame gets delivered directly to your home in a more remote location.