Firstly I doubt that an incorrect number makes any difference to ranking whatsoever. But it is a strange problem.
Did you add some posts directly to the database? Try going to Admin > Stats and clicking the buttons "recount posts" and "recalculate categories". You should then get correct numbers.
If not then it's hard to know the problem. Is the number 64 increasing when a new question is asked in that category? Are there questions in that category directly and not subcategories?