As @Thanos Legion @narniadis said its not estimated by Mojo but they source it from NATO.
But I find wrong is they overestimate admissions and their inflation adjustion is faulty. Inflation adjustion shall be done using ticket price index not simply on basis of (their estimated admissions * ATP) as they do.
e.g. Jurassic World did $652,270,625 in 2015, so it's business in 2018 shall be
Gross (Actual)/ ATP of 2015 * ATP of 2018
i.e. $652,270,625/$8.43 * $9.14 = $707,206,822 as compared to $722,509,700 they gave.
Similarly Avengers (2012) shall be 715,765,238 not $702,694,300.
Their criteria is against those films which have higher ticket than ATP of that year and favours those which have lower ticket price. For adjustion they consider same price for both i.e. ATP of current year but in reality the film which coated higher than ATP in the year it released will cost higher this year as well.
After I have typed all this, I don't know if I am making sense to you all or not because in my head it's very clear.