Hi philosophers/ontologists/epistemologists

I have a question about supervenient physicalism.

I feel I understand the logic of supervenient properties in regards to things like morals and aesthetics.

Example - A man decides to kill another man

The killing is = A
The reason for killing is = B

The reasons for killing can change, however the if the person is killed or not is whether A state changes.

What I dont understand is how it is applied to physicalism.

If you subscribe to the idea that brain states equate to mental states 1:1 - how could brain state A (the way atoms are arranged, neurons firing ect) be the same but the B state (what you are feeling psychologically, phenomenologically etc) can change without A changing.

Does this make sense or does someone need me to explain better?