Marriott has stated over and over that amenities and how nice the hotel is has nothing to do with the category. It's all about redemption rates. When a hotel gets a lot of redemptions they jack it up a bit. Not enough redemptions, they dial down. They seem to have some target for redemption percentage and they will fiddle with the category of the hotel until it hits that target. Some hotels seem to bounce around a lot, probably because at a Cat 5 the hotel got a lot, so moved up, then at 6 not so much, move down, etc.
Usually whenever I feel like a hotel is a real steal it goes up. This has happened at the last 4 or 5 hotels we booked a 5 to 7 day stay at!