When searching for a hotel in Bridgewater, NJ for example, there are over 5+ Marriott properties within 5 miles. The room rates are all tightly concentrated between the various brands, all between $90 and $99 per night. However, when using points, the range is 10,000 to 25,000 with no discerning trend between properties. I have always seen that points generally run 10,000 per $100 of room cost, but every so even such as this example see wide variations. Only reason I can think of is some properties do not want to redeem points and discourage this by jacking up a basic property to 25K.
There are a lot of factors that go into it. Could it be that the $90 is a minimum price for the time you are looking, but at peak time the better hotels are MUCH more expensive than the worse ones? For example, the Marriott Dulles Suites is a Cat-4 property, requiring 20,000 points per night. On weekdays, the room rates are $279. On weekends, they are $90 or lower. I have even booked them at $75 a night! 20,000 points for a $279 room is a great value, but for a $90 room, it is a terrible deal. Other marriott hotels in the area may hover in the $90-$150 range. At times, they are all the same rate--the value of a room. Other times, they are very different.
like ndn mentioned, it may be a low occupancy days. from looking at the properties the higher point properties(full servise marriotts) might be seen as more luxorious with more amenities such as restaurants and lounge as oppose to courtyard or residence inn. As for pricing, full service marriotts may price their rooms competitively to draw in more people knowing you will be more likely to dine at their restaurant, pay for internet and other amenities which gives them significantly more revenue. Smaller properties may not afford to go any lower than that for maximum profit. The rates may vary widely during peak days and assume the full service marriott would be priced much higher. Also higher point redemption has to do with how popular the property is with rewards redemptions, so i assume the 25k one is more popular with people? why? because its a full service marriott.
Remember also that categories are assigned hotels by how popular they are (and as many posts here on MRI have alluded - # of guests using points to redeem stays). But ndn nails the explanation - low occupancy will drive down the ca$h required, but not the category.