A 365-day deposit and a 1-year certificate at the same rate: why do the returns differ slightly?

Usually because of the calculation method, not the rate. A deposit's return is usually computed on the actual number of days divided by the number of days in the year the bank uses in its terms, while a certificate is computed in months over 12.

Example at a hypothetical 20% per 100 pounds: a 12-month certificate gives 20 pounds. A 365-day deposit on a 365-day year also gives 20. But if the bank counts the year as 360 days, the same deposit gives about 20.28. Over shorter terms the gap shows more: a 182-day deposit on a 365-day basis gives about 9.97, and a 6-month certificate gives 10.

Another possible source of a difference is the day each product starts accruing the return.

So when you see a list mixing certificates and deposits, remember the calculation basis differs and a small gap in the return is not necessarily a gap in the rate. Check the key-facts statement for the day the return starts and the number of days per year used. If you still see an error in your account, ask the bank for the detailed calculation, and you can file a complaint if the answer does not convince you.