The figure I've seen is, 40% of the registrations have been discarded by ACORN itself, and another 30% have been questioned by the election authorities.

That sounds as if it means that 40% of registrations were viewed as suspicious by Acorn, and were duly supplied to the registration authorities marked as such (in line with the law). And that subsequently one in four of these were allowed to stand as valid registrations by the registration authorities, after they'd looked into it.

Maybe that's how things happened, and those figures were correct. Or maybe, of course, those figures were just dreamed up by someone.