The difference is due to the way the API and Currency Converter handle dates. The API returns either the most recently published rate or the rate for a specific date. On the other hand, when asked for a specific date, the Currency Converter returns the rate for the previous day or the most recently published rate before that date. When comparing rates, please keep this in mind when setting parameters. For example, if you want to get the average rate for Apr 1, you should set the parameter as Apr 2 on the Currency Converter page.