Nobody can close a working kitchen to count it, so a count session has to tolerate a store that is still moving while it is being counted.
The rule that makes that safe is that an uncounted line is uncounted, never zero. A null persists as a null and is skipped entirely on completion. Coercing it to zero turns a half-finished count into a property-wide write-off of everything the counter never walked past.
The correcting movement is computed against live on-hand, not against the figure captured when the session opened. Stock may legitimately have moved since — a delivery arrived, a breakage was recorded — and using the snapshot would log a fictional delta while forcing the cache to the counted figure, so the ledger would stop summing back to the number on screen.
The sheet shows both figures for exactly that reason, and reopening a completed session preserves the quantities actually submitted so the corrector edits real numbers rather than starting again.