Agreement is a weak test
Reconciliation asks whether two totals match. It does not ask whether either total describes reality. When both systems draw on the same flawed input, they agree precisely because the error is shared.
The useful question is narrower and harder: does the combined record describe a sequence of events that could actually have occurred?
Where the failure hides
Exposure accumulates in the space between systems, because no team owns that space. Every department defends its own numbers and every defence is honest.
An investigation begins by refusing to accept any single system's account as the account.
PERISCOPE briefings describe investigative principles. They do not disclose detection logic, thresholds, formulas, weighting or rules.