A load-voltage-is-the-sum-of-visible-arrivals ledger adds the load-node deltas from the arrivals that have actually reached the boundary. The 12 V first load change plus the -3 V next change gives a visible 9 V total only for the stated ideal bounce sequence. Real measurements include attenuation, dispersion, source and load tolerance, connector discontinuities, trigger timing, finite precision, and measurement uncertainty.
Load voltage sums load-boundary deltas, not raw wave arrows. 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 load arrival includes its own reflection
The first load arrival is 8 volts, but the load-node change is 12 volts because the load reflection is included.
8⋅(1+21)=12
The next load arrival subtracts from that total
The next forward arrival is -2 volts, so its load-node delta is -3 volts.
−2⋅(1+21)=−3
The visible load total is the partial sum after shown arrivals
The shown load-node deltas are 12 volts and -3 volts, for a visible total of 9 volts.