Thank you, Goldorak, for raising the issue. This has annoyed me for some time, too. Recently, I was looking for a hotel room at Novotel NYC but wasn't able to find the advertised price. I contacted the hotel and inquired explanation. This is what I got as a reply:

"These prices are the 'starting from' prices. They correspond to the lowest average price available on the dates requested, based on a room for one or two people. They may be based on different room types. Depending on the country, these prices may not include taxes, may include VAT only or may include all taxes (VAT and city tax). When prices do not include all taxes, the relevant taxes (VAT and/or city tax) will be stated in the following stages of the booking process. The price is only guaranteed at the time of reservation."

The difference between the advertised and the actual lowest price was so significant that I decided to book Airbnb instead. The advertised price was 229€/night but the actual lowest price was 316€/night, so the difference was 87€/night. For a five-night stay that made 435€.