The vast majority of NFL tickets are sold before the start of the season so teams are still claiming sellouts, which while technically correct, gives an impression that isn't reflected in actual people in the seats. I'm not sure how the NFL counts attendance but in NCAA football each school decides on what methodology to use. Some don't count hotdog vendors, some count every person inside the stadium, some just estimate the crowd without bothering with an actual count. It wouldn't surprise me if some pro teams massaged their numbers to appear larger than the actual number of fans who show up.