Ok, I'm going to talk out of ignorance, but I have 2 properties that this has happened at. The explanation I have gotten is that PMS (Property Management System) does not interface with MARSHA (Marriotts Reservation System) in a LIVE fashion. It downloads batches, and in doing so, has produced these four situations:

1. I book a rate, a week later see a lower rate and modify the reservation, then check in 2 weeks later. At check in, they see the original rate, go into MARSHA, see I am correct, and update my rate.

2. I cancel the reservation 48 hours ahead of checkin (that was subject to cancel by 6pm day of arrival). I was still charged the penatly.

3. I changed a reservation a few days before arrival (that was subject to cancel by 6pm day of arrival) to push checkin date back 2 days. Upon arrival, the property had canceled my reservation and charged me the no show penalty.

4. And just today, I changed my checkout date when I checked in. (6 nights instead of 7). I checked out after 6 nights, and still see on my efolio/credit card, that I was charged for 7. (We'll see if they fix this without a fight)

The first three just caused hastles, but were all resolved. I am learning to call the property directly and demand the front desk (who want to transfer you to in house reservations) and have them look and verify the changes in there system well in advance of arrival.

YMMV