This has nothing to do with "real life". It's simply a gameplay definition, and it only has a meaning in terms of score.
In RL, hotpadding was sometimes used too, though of course it wasn't as quick as in Aces High. For example, late in the war fighters taking part in the Defense of the Reich would often land to refuel and rearm on a different field to attack the bombers again on their way back home. I don't know if those were counted as separate sorties, or if it did even matter at all.