It's a big problem with the OTA industry the price advertised is not always the price you pay, most of these sites allow the Host to add additional fee's and you as the guest are not aware until you book, To some of the major hotel groups it's what they call up selling check out some of the mad things they will charge extra for like Valet parking,
In the last 12 months Airbnb have tried to address this problem so before you book you are aware of any additional fee's not added afterwards,
The biggest problem I have seen over the year many guest do not read the full listing and what additional cost, such as cleaning fee, pet fee, electricity not included,