Changing the Number of Guests for Airbnb in Okinawa

I made a reservation from January this year for my family trip to Okinawa, Japan in October. Upon making the first reservation, I only paid around 43,000 yen for five of us. However, my oldest daughter couldn’t join us so I needed to change the numbers from five to four so as to save money. I had to pay about 18,000 yen for this change. I have tried to solve this stupid problem with the host and the host agreed with my complaint that I didn’t have to pay. He also said he didn’t understand why should I pay more and did not know how to make the change. He advised me to contact Airbnb directly. However, Airbnb insisted that the host had to agree to give me a refund. Is it normal to pay more to decrease the number of guests? What a horrible policy! I wouldn’t use Airbnb again.