I synced my Coinbase wallet to do Tax Report and it's showing yellow exclamation mark "GBP balance doesn't match what you have on your wallet. The balance calculated from imported transaction for this coin is HIGHER than your actual balance by 0.2 GBP".
I had a detailed look at transaction history on Coinbase and what Koinly imported via their API.
Coinbase is rounding GBP amount it deducts from GBP balance to 2 decimal points for each "fill" in advanced trade.
However, it's reporting reporting many more decimal points via API.
Koinly is adding up GBP cost of fills first and then rounding for the whole advance trade.
This different rounding causes the fiat balances in Coinbase and Koinly to be slightly different and it may increase over time.