this post was submitted on 13 Sep 2023
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It's not per kilogram of the device. It's a measure of absorption per kilogram of body mass of the person using the device.
A good way to think about it is: throw a small piece of meat into the microwave and let it absorb some microwaves. It gets warm. Now imagine a much larger piece of meat. It can absorb a lot more microwaves before it starts getting warm. The smaller the 'body mass', the less radiation it can absorb and dissipate before becoming damaged. So when it comes to mobile device radiation levels, they focus on the watts of radiation absorbed per kilogram of body mass.
and different parts of the body resonate at different frequencies...~~part of the reason ~2.4 GHz was picked for the common household microwave is water molecules resonate there~~ (and other harmonically related frequencies too... it's why a lot of the unlicensed ISM stuff is allocated there: crappy atmospheric propagation). also the necessary magnetron and waveguide for that freq is conveniently sized for a kitchen appliance and not too complicated.
EDIT: see proper principle of operation in reply below