I have the same issue and just called their customer service. I got 35Euro on an Android Tablet from 2012, 49 on Chrome on Linux and 57 a night on iOS 11 with a 2017 iPad Pro. It's interesting to see the correlation between the likely price of my device / my financial capability and the pricing of my offers.
Anyways the recommendation of the customer service was "please always use chrome to make sure that doesn't happen. The other Browsers are somewhat buggy". Why the iOS app reports higher prices wasn't answered.
To me that is a clear pricing discrimination and will keep me from booking with AirBnB in the future. And if at all, best to use a Windows XP machine with IE 9, so that they at least think you don't want to throw money out of your window.