Do I have to make up the difference when booking a hotel in advance on holidays?

No need to make up the difference. During the peak period of holiday travel, many hotels will raise prices during this period. Usually, when booking a hotel online, you will be prompted to pay in advance, because this will ensure that the house price will remain unchanged during your stay, that is, the hotel will successfully pre-sell the room. The earlier you book, the cheaper it will be. If you don't book in advance and enjoy a cheaper price, the price will rise temporarily. This is not the spirit of contract. Booking a hotel in advance on holidays does not need to make up the difference. If there is no advance payment for online reservation, the reservation will still exist at the time of check-in, but the hotel will inform you that the room has been sold, only the high-priced room, because if the reservation is not sold, the hotel will generally not keep the room for a long time.