The Ninja troops have embeded in their code under AI control (on defence) to “sometimes hold charge to higher levels”
Meaning that they will sometimes hold their charge to fire at 2x or 3x rather than inta-firing at 1x all the time.
From the looks of things, Garnet was meant to hold until 3x charge…
At a guess, I would say that MAYBE (in an attempt to be more human-like) there is a “failsafe” or “emergency” trigger, where if the HP is low, it over-rides the pre-determined charge level & simply fires if available when under “Low HP” condition…
BUT I’m not staff or QA so that’s just my guess.