The "traditonal" holiday charter carriers, who have lost out over recent years to the low-fare operators with more balanced flows, have long been very adept in finding means to minimse empty legs. Having the Mediterranean summer charters begin just as the ski season is ending helps; around Easter there can be triangular trips, say Manchester-Palma-Geneva-Manchester, whch take out the first summer tourists and bring back the last skiers, with only the middle leg being empty.
In addition to the holidaymaker pax, there is also the need to get reps and resort managers out at the start of the season and back at the end, which can require some enhancement of seats.
Any charter operator has inefficiency to absorb by the nature of their less-regular operations, whch is why, when someone has say a group of 200 to move as a one-off, they can be surprised to find it is more expensive than regular flights, because there is no backload available. Military charters are a particular case in point.