6 months out many hotels will already have a pretty good idea of their saleable inventory because:
MICE market almost always books >6 months out
Agreements with tour operators/agencies are usually signed 12 months out
Corporate agreements may stipulate minimum occupancies (although this is unusual)
Government agreements are typically tendered once per year- so the hotel will know what level of business to expect here
Leisure travel/Corporate independent has trends that can be analyzed year over year
Events, holiday weekends, etc are factored into the equation
Room blocks usually open up @ 6 month mark if not before
Thus- the hotel can project with reasonable accuracy what discrentionary inventory they will have to sell- which is exactly where hotwire and other non-transparent channels can fit in.