Hotels in Japan have slightly different systems than the systems in Europe. For example, there are more twin rooms than double rooms. Sometimes European do not understand the difference between twin and double rooms.
Twin room is a room which has two beds. On the the other hand, double room has only one bed. Two persons will sleep separately on each bed in a twin room but sleep together on a same bed in a double room.
Then, usually twin and double rooms are the same rates in Europe, but usually twin room is expensive than the double room in Japan. Sometimes hotels in Europe have same rates for 1 person staying in a double room and 2 persons staying in a twin room, hotels in Japan mostly have different rates between 1 person or 2 persons staying in the same room even if the breakfast is not included. Of course 2 persons staying in a same room is expensive than single person.
Therefore if you make a booking, please carefully check your room and choose the suitable room.