Arsenal also count season ticket holders even if they don't attend the game, there's no way they got 60,000 every home game last season. My uncle is a goon ST holder and he reckons there were probably around 50,000 at times.
It has become the fashionable thing to do. United first did it, and now every big team does it. Attendance is the wrong term, as clubs clearly only count tickets sold. Our attendance in European games is nothing better than poor, but in league games it is always close to being full.