Is it just a matter of bigger numbers? Covid limited crowds last year, so maybe more people came, and does falling on a Sunday mean all those who would usually go before/after work came during the day?
Definitely difficult more people because it's on a Sunday (last time was in a Sunday was 2008) and a number of other factors.
They really didn't have a way to know what to expect.
2020 - Covid
2019 - Shabbos
2018 - Shabbos
2017 - Tuesday
2016 - Shabbos
2015 - Shabbos
2014 - Tuesday
2013 - Tuesday
When it comes out on Shabbos there are obviously significantly less crowds and they are split between Thursday, Friday and Sunday.
If they were using the numbers from 2017, 2014 and 2013 (Tuesdays) to estimate how many people would come this year (Sunday), they obviously would not get anywhere close to a good guess.