First all: I love this option. For me it really adds value to park at a gate that is intended for the livery I am flying at that moment !!
This has been discussed too many times already, and of course is explained on the manual: the airplane YOU are flying doesn't have any effect on the choice of ground operators. ONLY the atc_airllne_codes in the parking, as defined in the scenery, do.
This is by design, and IS correct because, it would be very unrealistic that your airline could have its own ground crew everywhere in the world, such a Lufthansa flying to New York having his german crew at JFK.
Instead, with the parking driven by the atc_parking_codes, it's more likely to produce a more realistic situation, because in the previous example, that Lufthansa airplane might want to go parking on a parking that has his own code together other codes that Lufthansa code share with, like United, so a United crew might show up.
The atc_airline_code YOU are flying (IF present in your aircraft.cfg), will only drive the choice of the ULD pallet you are going to use. And of course, this is made intentionally too, in order to not having pallets on arrival different than pallets on departure.
