The attendance figures in question are based on the number of fans at the games, and the percentage is based on the stadiums' seating capacity. When the Bills were setting attendance records in the early 90s it was based on the number of fans at the games. The teams with over 100 percent are counting standing room fans in addition to the fans in the seats.
Interestingly, the Bills are the lowest attended road team in the league, no doubt in part because 2 of their road games were at Cincy and Miami.