That doesn't make sense. I have always been led to believe that the information is directly from the BTB records from the monthly statements from each hotel. This report includes the register of the guest's name, where they are from, how long they stayed, how much they paid per night and how much (9%) hotel tax was collected and remitted.
I always figures the stats would run lower than reality because I would imagine that some hotels, mainly the smaller ones, probably don't report all of their income.
The Belize Tourist Board doesn't have "members" per se. It is a government body that governs tourism. Every hotel is supposed to apply for and maintain a hotel license and every hotel is required to send in a monthly report.
I think you must be thinking about the BHA and BTIA which are private member groups.