Originally Posted by Goldorak

This is not the same situation. You are talking about booking on a corporate booking engine which may propose lower fares than the hotel itself, because they negotiate global volumes and deals with hotel chains to propose them to their customers. Same with sites like expedia, hotels.com, etc.
The OP was talking about cheaper rates on individual Accor brands web sites (e.g. sofitel.com, novotel.com, etc) versus the global Accor site (accor-hotels.com).