I'm asking this from a place of genuine ignorance: how does the evenness of the heat distribution matter when microwaving a pure liquid? I'm familiar with the microwave's uneven heating qualities. I'm sure we've all bit into food that is scalding hot on the surface and still lukewarm at best in its interior. However, I've always presumed that is a product of microwaving a heterogenous, predominantly solid substance.

So, sure, the microwave applies heat unevenly to the water. But wouldn't the tiny little bits of water which get "over" heated simply diffuse their excess thermal energy into the rest of the homogenous volume in very short order? Furthermore,wouldn't an uneven heat distribution in a mug of water simply lead to convection currents flowing from hot to cold, therefore promoting a relatively even distribution?