Follow-up on that one: The table on the website already lists averages until the specific month, i.e. the exchange rate for March is the average exchange rate from Jan-01 to Mar-31. So that’s why one has to use December’s rate (because it’s the average rate from Jan-01 to Dec-31).
The only exception to this seems to be the calculation of AMT for basic income: Here, daily exchange rates should be used (while the tax bureau remarked that if these are “difficult to find”, using the average might also be acceptable on a case by case basis).