Leakage is an erasure budget that carries lost counts separately. Exact arithmetic here means exact results for the stated model inputs; measured inputs still carry uncertainty and significant-figure limits.
highlighted = computed this step
Leakage splits each state into kept and lost counts
From the channel output, zero has 80 shots and one has 20. One fifth leaks away, leaving 64 zero and 16 one shots.
Nkept=(64,16)
Lost shots leave the computational readout ledger
The lost counts are 16 from zero and 4 from one, so leaked shots total 20.