As tickets are Sold out months in advance to the date of the particular match. But then on that day of the match attendance could fall from fans, either to anger, a boycott called against the owners, or two matches being played in a few days apart, fans would prefer the watching the more important match. So lets take this match for example. In the Europa League match played between Arsenal and Ostersunds. there were very few Arsenal Fans at the stadium, and a joke went around that there were more Ostersunds fans then Arsenal fans. But in reality the club count the number of tickets sold out for that match on that day. And please note they do not go around counting the seats which are empty.