Ah, it does make sense though. Sirin was a random girl from Siberia who just got the cores, while Kiana was a Kaslana and Schariac. Both of which have a high amount of Honkai Energy level tolerance which would increase the cap needed for a Herrscher to reawaken, if you remember, Otto directly injected Honkai into Kiana even after injecting the core of death into her. Plus, HoV herself could've drawn on the Honkai Energy from the tech that Schicksal uses, and all kinds of other things. And if you remember, when Kiana was 12, a Honkai Eruption occured and HoV partially awakened, it's likely that she absorbed the Honkai Energy of that Eruption into her core which would stack ontop of all of the other Honkai Energy.