I haven't noticed exchange rates that often being significantly off in the majority of cases, but once or twice in the last year I have had similar issues but they have been resolved by customer services, this is when the delta is 60-80%. I remember a hotel in Fiji had this issue last year for me, but this was on room rates, not just incidentals or small spends.

However, when I have done some deep dives on more 10-20% differences it has often reminded me of non-earning things like service charges (10% of charges - 10 ~ 15% service charge when booking Asia properties direct from Marriott website?). There is a good thread on this here: https://www.flyertalk.com/forum/marr...rn-points.html