Discrepancies in Bilateral Trade Statistics

For bilateral trade flow, there are usually two sources of data from
1. exporting country,
2. importing country

Intuitively, the exports reported by an exporting country should mirror the imports reported by its trading partner. In reality, it rarely the case. Why?

Reasons based on the study by various international organization.

1.Time lags
2. Valuation
3. Types of goods excluded from trade statistics
4. Trade System
5. Extent of re-export
6. Methodology difference
7.Exchange rate
etc


Some of the studies compiled:
What's the difference?-Comparing U.S and Chinese Trade Data
Discrepancies in Bilateral Trade Statistics: The case of Hong Kong and Singapore
Mirror Statistics , UN
Difference in the Mirror Statistics in INTRASTAT, EU
Why are imports and exports different? ITS, Newsletter
Adjusting Chinese Bilateral Trade Data: How Big is China's Trade Surplus? IMF