The main reason you would be given an upgrade is a maintenance issue with the room you are scheduled to be in. If you ask for and receive an upgrade at check-in the person you are renting from will mostly likely be charged the extra points for the upgrade. That being said, I am one of the few to have received more than one upgrade:
1. Once cause of over booking at VWL:
Booked a studio at VWL, when we tried to check-in, we had been upgraded to a WL 2 bedroom lake view suite. Called some family members to come and join us for an unexpected vacation.
2. Once cause of problem with check-in at AKL:
Booked a standard view studio at AKL-Jambo, checked-in, given a room key as the room was suppose to be ready, went to the room, and found suitcases and purse in the room along with the room not being made up. Went back to the front desk, they gave us an on-the-spot upgrade to a savanna view room in another part of the resort plus four free breakfasts.
3.
DCL
We reserved an inside stateroom but were upgraded to an ocean view room. We did not find out about the upgrade until we got to the room, what a surprise that was.
4. AKL-Jambo
Booked standard view room at 5 days out, due to a last minute change in home plans. Given a Savanna View room.