I think for most user stats, the delay is just one day, but for counting the overall number of users, I think there is another source of discrepancy: maybe the user stats page only counts those users that actually generate measurable stats. Some accounts are maybe dummy/system accounts, while others may have created an account, but then immediately left the forums without generating any stats.
By the way, @Atorrante, you gave me my 10,000th like on this forum (on the post above).
Edit: and @Gia got 9,999 and 10,001.