I think it's fairly common to equate attendance to ticket sales, and 30 years ago it was probably a perfectly adequate measure, but it seems rather pointless in the modern era. Historically every ticket sold would more or less be used, but in our day of affluence and tens of thousands of season tickets sold it no longer tallies.
Especially with the technology of today; think of Spurs who must know exactly who has had their ticket scanned and who hasn't. It's not a matter of manually checking them, so a manual count would be needed, it's literally scanning every ticket where a computer system counts them off. Woolwich I believe have the same set up.
I guess it's all political isn't it. If we and they have to report on actual attendance but Sunderland (who, for the sake or argument, lets say don't have the scanning system) don't, it would skew the numbers. But at least stop calling it "Attendance", call it something which means "Tickets Sold"...