the claim is that they don't hold any back, except for special arrangements like the Candlelight Processional package tying up a fair number of Epcot reservation possibilities right now. Obviously they can't book up all the CP restaurants with regular reservations - then they'd have no tables to offer for the CP package when they start booking those.
...but there are constantly cancellations going on. I know I book, change my mind, cancel & book something else 2, 3 or more time every trip. And sometimes, it's more than one place I do that with, too. Read these boards & you'll get a good idea how often people change their minds & cancel stuff --even what you'd think was very popular places.
also, the closer it gets to the 45-day-final-payment-due-date, the more cancellations you'll see as some people will decide to cancel their whole vacation, too. One of the things they ask when you call to cancel a vacation is "Can I help you cancel your dining reservations, too?" They don't want people getting upset later if they get charged for a no-show when they cancelled the whole vacation.