Probably the biggest difference between Japanese hotel and European hotel is the number of double room. Japanese hotels have plenty of single rooms and twin rooms but much less double room. On the other hand, European hotels have many double rooms but less single rooms and twin rooms.
Even people in Europe do not acknowledge the difference between double room and twin room due to single name is used for both rooms in their own languages. Actually double room has one double, queen or king size bed but twin room has two single size beds.
Japanese couples prefer to stay at twin rooms, especially if the couples are middle age or elder. On the other hand, European couples prefer to stay at double rooms even they are in middle age. Therefore hotels in Japan do not have plenty of double room which are requested for European guests.
Then, the room rate is usually different between twin room and double room. If the rates are different, twin room is expensive than the double room. On the other hand, it can happen that twin room is larger than double room in room size.