When an user check in to a beer on Untappd and select festival venue, Brewer Stage can capture the rating score. Also when user uses in comments tags that identifies the festival such as FBC2018, Brewer Stage can also capture the check in.
The top lists (beers and breweries) uses a bayesian average model to sort items by average of rating scores. This is the same model used by movies in IMDB.
Besides that, only one score is used per user per beer to calculate the average. There are also filters that blocks fraudulent users.
The minimum number of scores for a beer be listed on ranking is 3 or 0.15% of festival total number of scores (the greatest of both, subject to the maximum of 15). A brewery needs 3 times the minimum number of scores for beers to be listed.