Hello guys,
I had the same problem too, my fixed price was 65, but then on my listing it was always changing, one time it was 70, then 85.
So I tried to figure it out and I noticed that the more guests I included (when I simulated a reservation for my place), the higher was the price. For instance, if I inserted just 1 person, it would be 65, but if I inserted 2 people, it would raise to 70. And if I put 5 people (maximum of my place), it would raise to 85! So I noticed that everytime I added a person, it would add 5euros to the fixed price. And I remembered that that was my extra host fee (in my listing definitions). An additional 5 euros for over 1 person.
Then I decided to change my definitions, and instead of over 1 person, I put 5 people (the maximum of my place). I then made the test and simulated a reservation for my place once more, and the price at last kept the same, no matter how many people I inserted (of course only up to 5 people).
So see if that is also your case, and instead of putting over 1 person, change it to the number of people that your place hosts and then simulate a reservation and watch if it stays the same, or if it still keeps on changing.
I hope I could help!