As stated earlier, it's to do with the method of fueling. At airports where there is an underground hydrant fuel supply, the fuel truck simply parks under the left wing and connects one hose to the aircraft and another hose to the fuel hydrant. You may ask why the fueller doesn't fuel under the right wing !? This is because the loading teams would be unable to maneuver the baggage container trailers with the fuel truck parked under the wing (BA use containerized loading on LHR based shorthaul fleet). Easyjet, for example use the 'bulk' loading method, and is easier for the loading team to position the trailers of bags. without causing conflict with the fueller. At airports that do not have an underground hydrant system, you will notice that the aircraft is refuelled using a Tanker which parks on the right hand side of the aircraft away from the loading team and connect directly to the aircraft using a long hose. This in turn frees up the left hand side of the aircraft for boarding/de-boarding.