“A GFCI only trips when the current flowing in the hot leg doesn’t equal the current flowing the opposite direction in the neutral leg”
Isn’t that what happens during a short? The whole point of a GCFI is to break the circuit when the toaster falls into the bathtub.
Not necessarily. If the short is from hot to neutral, the GFCI will not trip. The GFCI is looking for current to ground, where it is not supposed to be. Water would provide that path.