And this is a theory:
Sativa and Indicas have different effects, Indicas are more Sedating while Sativas are more uplifting or stimulating. But both of their main active ingredient is THC (some contain CBD and THCV, but those aren't in all or really even many of them). Lemonene has proven to be a mood enhancer on its own, Myrcene is a Analgesic on its own and helps THC cross the blood brain barrier so could make it stronger. Sativas are more fruity, Indicas are more spicy or woody (for the most part).
Then there is a second factor that could be creating these effects, the activation and deactivation of different CYP enzymes. Enzymes activation and deactivation could completely change the way THC is metabolized.