Hello Hwansoo,
It's normal for the guest to pay a higher price than what the host sets.
Airbnb adds charges to the ammount you set.
The charge they add is a % of your charge and it can also include currency conversion fees if the guest lives in a different country to the host.
In my case I charge €25. The guest usually pays €28 and Airbnb pays me €24 as they take a fee from me also.
If this doesn't explain your issue then you should contact Airbnb if you feel you haven't been paid enough.
The guest should contact them if they feel they have been over charged.
Steve.