Why the number of receipts received by the API differs from the number of receipts in the Back Office for the same period.

[Andy]

In the API every receipt has 2 dates:
created_at - the date, when the receipt was received by our server.
receipt_date - the date, when the receipt was created on the POS device.
Accordingly when a POS device was offline for a long time, then the date of receipt_date and created_at can be different. Filters created_at_min and created_at_max in the API filtrate by date received a receipt in the server. In the Back Office, receipts are displayed by receipt_date, the date when they were created on the POS device. 
 
Also, another reason why the number of receipts differs for a specific date in the back office and in the API may be related to time zones. Receipts in the back office are displayed according to the time zone of the PC on which the users are checking them in the back office. And in the API, receipts have a date according to the UTC +0 time zone.
 
So, if you need to build analytical reports for certain periods in a third-party system, then you need to use the date of receipt_date, not created_at, and also take into account the time zone.
https://developer.loyverse.com/docs/#tag/Receipts