@Helen3 is absolutely correct.
Historically, the guest fees were advertised by Airbnb as 6-12%, then in late 2017, they went to 5-15%, and in April of 2018, the relevant information on the site (and in the T&Cs) was updated as follows...
Guest service fees
When a reservation is confirmed, we charge guests a service fee between 0% and 20% of the reservation subtotal. Guests see this fee on the checkout page before they book a reservation.
In areas where we’re required to collect VAT, well combine the service fee and VAT amounts on the checkout page, so the service fee may appear to be greater than 20%.
Guest service fees are calculated using a variety of factors including, but not limited to, the reservation subtotal, the length of the reservation, and characteristics of the listing. In general, higher reservation subtotals have lower guest service fees.
Understandably, this hasn't gone down at all well with guests, many of whom found themselves shelling out up to 23%+ in services charges on their bookings, and they've made their feelings known on that in no uncertain terms.
To make matters more confusing, since April 2018, Airbnb has also being trialling several versions of the "Guest Pays No Fees" model, running concurrently, across different markets, for listings in both the professional and regular host sectors. (This is Airbnb's typical MO in advance of rolling out its most unpopular policies universally - create confusion as a diversionary tactic, then amid the chaos, everyone wakes up one morning to find the hated policy is a done deal)
Traditionally, Airbnb has chosen not to show hosts how much their guests are paying – presumably to prevent hosts from being shocked by how much money they're missing out on, which could lead hosts to cut out the expensive middle man and go for more direct bookings with their guests in the future.
When pressed for clarity on its opaque, ever-fluctuating service charges, the company's response has remained largely unchanged..
“We are constantly testing new and different ways to help our hosts accommodate more guests. These small, temporary, and voluntary pilots are some of the many experiments we are running as we try to learn more about how we can best serve our community.”
This led directly to the EU Commission introducing new legislation which came into effect on Jan 1, 2019, compelling Airbnb to present its pricing - including fees and service charges - in a clearer and more transparent manner, across all EU language versions of the site. Full compliance by the company appears to be a work in progress.