In 2018, MK had 34 Halloween parties and 23 Christmas parties. Based on the piecemeal information I've been able to gather, there were also at least 13 After Hours events and weekly-ish(?) Early Morning Magic events for most of the year. Depending exactly how many of the non-events there were, that essentially increases the number of "days" MK takes admission by 1/4-1/3 over the competition. Even if daily attendance is flat, they continue to grow annual attendance by adding days to the calendar.
Or do we really believe that MK averaged over 57,000 people every single day in 2018, when the park's capacity hasn't significantly changed in nearly 3 decades? I'm sure many days were at or above that number, but I have a hard time believing that's the average.