M.J.
I think it has been noted on other threads--WT writers are occasionally sloppy or lazy or both when it comes to research.
Probably what happened was some sub-sub-sub-lackey was asked to look up worldwide attendance for the 1935 memorial, found "a" number in the 1936 yearbook, and the number went up the line to the final writer. No one on the editorial staff bothered to verify it, or else the editor made the same oversight as the lackey, and the number slipped into the text.
In the 1996 article, some other lackey was told to get the same number. He had a different source than the 1988 lackey, and so that different number was printed in that article.
There's a thread somewhere on the board from Barb Anderson, regarding the infamous "work that will end in our 20th century" statement in the 1/1/89 Watchtower. Some semi-looney WT writer slipped his personal opinion into the article, and no editor caught it. It was finally caught and changed in time for the bound volumes and the CD.