The overheated particles will rapidly move upwards, which will lead to relatively even distribution in a layer, but uneven between heights.
In fact, in a large microwaved mug the difference between top and bottom can be as much as 6°C/11°F.
Using a kettle mitigates it for the most part, as it is the bottom that gets continuously heated, and the top is then naturally heated by the vertical currents of hot water, leading to a more even distribution.
The overheated particles will rapidly move upwards, which will lead to relatively even distribution in a layer, but uneven between heights.
In fact, in a large microwaved mug the difference between top and bottom can be as much as 6°C/11°F.
Using a kettle mitigates it for the most part, as it is the bottom that gets continuously heated, and the top is then naturally heated by the vertical currents of hot water, leading to a more even distribution.