AB used a combination of the % change numbers reported by the companies, and a few other factors, like going out on random days and counting cars entering the parking lots and such... To "guess-timate" annual attendance.
Universal seemed to "hint" to AB what there attendance was since they'd make statements like "AB is in the neighborhood". On the flip side, Disney kept saying the AB numbers were off, and getting worse off every year.
In the end, the numbers were of no use except to us on-line geeks that liked to use them to show whatever we wanted the numbers to show.
That said, someone needs to at least keep track of what's revealed in the various SEC 10K reports and tally a running report on various parks' attendance. I'd be willing to set up a page for that and take one company if other readers or webmasters would agree to follow others. I'll make it a Creative Commons page, too, so that other websites could use the content, so we won't have the copyright issues that we used to have with running AB's entire list.
Volunteers? (People with CPAs or experience reading SEC reports strongly preferred here. This ain't light reading for someone who's not looked through a corporate financial statement before.)