If you arrive at a convention hotel without a booking in the middle of an exhibition and there is a room available, you will be paying more than the regular tariff. On the other hand, many inner-city hotels that rely on business travellers have cheap weekend rates on Saturday nights designed to attract customers who would not be interested in staying otherwise.
The hotel industry has long been aware that customer demand for its product varies, depending on the time and day. As a result, hotel tariffs, which are designed to maximise both revenue and occupancy rates by pricing along customer-demand curves, change. In a nutshell, this is "price optimisation" - adjusting price according to the customer's willingness to pay, or demand-side pricing. The same is true of the airline industry: it is rare to sit next to someone on a plane who has paid the same price as you.