Just rang the club, the ticket exchange is operating, which is why tickets have been popping up all over - they probably just haven't been able to get it to work online.
I think it was measured by turnstile clicks, tho it's an interesting point because 'big' matches (ie likely to be 30k) were 'all-ticket'. I wonder if they counted tickets sold on those occasions?
Arsenal have been doing it ever since they moved into the Emirates, which is why their attendance is always 60k despite there being masses of empty seats. I think it's a shame as it makes comparison with old records pointless - the 'attendance' should be exactly that, not the number of tickets sold.