I think it has to be noted that Crew also have several different responsibilities aside from cabin service after the main service/s have been completed.
Certain positioned Crew may have bar reconcilitation duties and or Bond forms to fill out between gally's and other paperwork which is their responsibility for the flight, so they may not have dissappeared but are just away working on something else, out of view. Crew who are in Assist positions or soley service roles may then be the "skeleton" Crew you see offering active service to the passengers. And when operating on a minimal Crew ratio this can make it very busy, especially on a 7 hour flight where it requires a full service and then a full replenishment/snack service (or another full service dependant on the Airline).