The open-system audit forces device channels to run before readout assignment. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
The device channel changes counts before readout
The prepared count is 100. The bit-flip stage changes the device ledger to 80 zero and 20 one.
Nafter channel=(80,20)
Readout assignment waits for the retained ledger
After leakage, assignment uses 80 retained shots, not the original 100 prepared shots.