I don't know. The simple answer is that I'm wrong and Bert is correct when it comes to estimating crowd numbers. He might have the precise, cultured eye of an aesthete, and I, the unrefined eye of a ruffian.
Or it might be as Pommpey suggests that the attendance is based upon bought tickets rather than bums on seats. If my memory serves me, I think Arsenal calculate their official attendances by this method, or at least used to.
Going back 25 to 30 yrs, it always seemed to be the other way at Bramall Lane. When the attendance was read out, it always seemed visibly to be a couple of 000's light.