An interference weight three-row scan adds path amplitudes of unequal magnitude before squaring, so the resulting probability depends on the checked sum rather than on either path magnitude alone. The three rows add (1/2, 1/2) for a full constructive sum of 1 and probability 1, add (3/10, 2/5) for a partial constructive sum of 7/10 and probability 49/100, and subtract (3/10, 2/5) for a near-cancellation sum of -1/10 and probability 1/100 for the stated toy amplitude ledger; real interferometers have visibility limits, calibration drift, and finite-shot uncertainty beyond this exact amplitude arithmetic.
Three path-weight pairs move from full constructive interference to a checked near-cancellation using unequal magnitudes. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Equal unsigned paths reach full constructive sum
Chapter three's H-then-H case is the boundary row here: two equal one-half paths add to an amplitude of exactly one, so the resulting probability is also exactly one.
21+21=1
Unequal path weights give partial constructive or destructive sums
Chapter six flipped a sign on matched magnitudes. Here the magnitudes differ instead: three-tenths and two-fifths combine to a partial sum whose sign depends only on whether the second path is added or subtracted.
Subtracting the same unequal weights nearly cancels
Subtracting the same two magnitudes leaves a small negative amplitude and a small probability, short of full cancellation because the two path weights were never equal.