Why can't I mimic my fiancé's voice to open Siri?

Often before bed my fiancée will set an alarm by speaking to Siri. As a sort of game, I sometimes see if I can mimic her voice well enough, saying exactly what she just said, to see if I can fool Siri. Laughter inevitably ensues as I attempt to parrot my fiancée's inflections, tone, and timbre. It's not perfect, but it sounds pretty good to me. And yet, nothing doing. What's the deal? Tl;dr What is Siri picking up that I'm not dropping?