This is a very good question. I have very often differences between my own estimate (using the calculator provided by Accor - for those who are not aware of it, see link below) and the actual credited by the hotel.
https://all.accor.com/loyalty-progra...index.en.shtml

Reasons for this can be:
- an honest mistake by the hotel
- some hotels trying to fool you by saying that the expenses at restaurant X are not eligible because it’s independent, while it’s fully integrated to the hotel and accessible only from the hotel and, of course, nobody told you about non-eligibility (Sofitel Dubai downtown is expert for this ).
- a wrong estimate by me of the taxes. Invoices in some countries are not detailed enough to understand properly the taxes.
- exchange rate issue, as described by Macaron95 above. For this, using the Accor calculator above, avoids this problem as all currencies are listed so I believe they are using their internal exchange rate.

I usually don’t act for a difference <50 points. Sometimes it depends on my envy to « fight » as it takes a significant amount of time.