I would assume actual attendance is off of scanned tickets. I think if you are sitting at the 100 level, the building appears more full just because of the angle you can view other seats. But if you are in the 300 level like myself, you look around the other 300 level seats and it can look a little sparse at times.
My guess is that no more than 60% of the 300-level seats were filled last night. I agree with Rosenthal's estimate. There just can't be 2,000 people in the bathrooms at any given time. Unless of course they had food poisoning issues in the concession stands.