monthly payment policy

Tell me, please, by what rule are monthly payments formed?
For example, if we take the period from January 1 to January 31, 2020 and count the daily reports downloaded from the currency on the day of the transaction, the difference with the total amount received on the account (bank card) will be about 10%.

The same big difference remains, if we recalculate currencies on the day of payment, then for January it was March 5.

Questions: On what day should we recalculate currencies, where to take the rate for conversion to U.S. dollars, maybe we need to count cohorts of 30 days instead of 1 to 31?
Payments reflect months start/end based on Apple's current fiscal calendar, which can be found here.

Example, new fiscal year just started, and Period 1 October 2020 runs from 9.27.20 to 10.31.20

Period 4, January 2021, runs from 12.27.20 to 1.30.21.

For 2020, I believe January ran from 12.29.19 to 2.1.20
monthly payment policy
 
 
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