A channel matrix maps input counts into output counts by applying declared rational row weights. The row arithmetic is exact only for the stated matrix and input vector; measured channel estimates carry sampling error, drift, crosstalk, leakage, and model-mismatch uncertainty.
A channel matrix is a declared exact map from input counts to output counts. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
A rational row maps zero counts
The zero row keeps 4/5 and flips 1/5. From 100 zero shots, that gives 80 output zero shots.
100⋅54=80
The same row also creates output one counts
The flipped share is 1/5 of 100, which creates 20 output one shots.