Reconcile two reports that disagree about the same number

Two sources report different values for the same metric and I have to explain the gap today. Metric: {{name}}. Source A: {{tool, value, how it is defined if you know}}. Source B: {{tool, value, definition}}. Period and timezone for each: {{state them}}.

Do this:
1. Quantify the gap in absolute and percentage terms.
2. List the candidate causes in order of how often they are the real answer: period or timezone boundaries, deduplication rules, filters and excluded populations, attribution or event timing, late-arriving data, currency or unit handling, and different definitions of the same word. Add any specific to the tools I named.
3. For each cause, give the one check that confirms or kills it and the exact cut of data it needs.
4. Say which source is more likely correct for this decision, and why - or that neither is authoritative yet.
5. Write the two sentences I can send to the stakeholder now, honest about what is unresolved.

Do not average the two numbers. Do not guess a cause and defend it. Do not invent values I did not give you.