Predictions are that, predictions. They are based on historical background, special events scheduled, and things like changes to holiday dates.
Disney does the best on its internal numbers, but they also spend the most time and money on it. They use additional info they get, including their own ticket sales, hotel and vacation bookings through Walt Disney Travel, but also through a semi-official group of most larger Hotels and other nearby Theme Parks and other large Tourist attractions. This information is shared on a daily basis and is used to help set advance Hotel pricing, and attraction staffing. The deal is they get the info in return for providing their own info, and as part of the agreement, that the information is not made public, though some folks have seen them, including people like the Unofficial Guide which run Touring Plans and other similar sites. Many times it is the predections made in advance, which does change and get more precise closer to the date.
And of course, you can never predict the weather, which has impacts, especially for the locals. A hot day, the Water Parks in SoCal sees an increase in business and the main Theme Parks see it go down. Also Southern Californians don't handle rain that well.
So as much as you can predict, something like an unscheduled school closing can increase attendance that day.
