Brad Redding (Connect on LinkedIn and Twitter) shares 12 reasons why he sees Facebook under or over reporting conversions. Details include day-of-conversion attribution nuance, reporting changes, tracking issues/bugs, and third party apps or checkouts. 

Under Reporting:

1. Attribution Window & Conversion Value Reporting Differences
2. Short Windows & Reporting Delays
3. Recent Events UI does not show iOS events
4. Missing fbc or other advanced matching parameters
5. Post purchase upsells not configured
6. Offsite checkouts missing CAPI
7. Facebook's own disclaimer 

Over Reporting:

1. Attribution Window & Conversion Value Reporting Differences
2. No deduplication 
3. Duplicate events
4. SPAM events
5. Offline conversion events API

Enjoy!

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Links Referenced:

https://www.getelevar.com/

Elevar Knowledgebase Article

Common Thread Collective article on iOS14 and Facebook Analysis

Post Purchase Tracking on Shopify