The channel acts before readout, so the readout assignment must use the counts that survive the device stage. In this pinned ledger, 100 prepared shots become 80 zero and 20 one after the bit-flip stage, and leakage leaves 80 retained shots for assignment rather than the original 100. Real devices add calibration drift, decoherence, leakage paths, readout assignment error, finite-shot noise, and uncertainty around this ordering.
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.