I remember seeing that category counts can be slightly off because a member page might have been blanked and restored causing the page to temporarily drop out of the category. It seems that the count can be off for other unexplained reasons. As an example https://en.wikipedia.org/wiki/Category:Days_of_the_year usually shows about 344 member pages. The correct number is 366. There are not enough regular page blankings to account for the difference in the true count vs. the displayed count.