Have you checked your Price Setting to make sure that Smart Pricing is not turned on? Looking at your listing you don't have any additional charges, no cleaning fees or extra persons so your base price should always be the same. However looking through your calendar, I note that there are different prices for different dates in particular last week in July there are days at £37, £40, £38 which suggest smart pricing.
Airbnb's fees are not in incorporated into the fee seen on the search page or even in you listing, they are provided as a separate breakdown for the guest after the total cost of their stay is calculated; their service fees have no impact on your nightly rate. If Smart pricing is turned off, then I would suggest you contact ABB to report the issue; click on link for contact options - Contact Airbnb
Hope it gets sorted soon,
Ama