The site shown a message saying I have been granted a privilege, but the reputation required for that privilege is higher than the one I actually have. What happened?
As stated officially here, this behavior is by design.
Your reputation was indeed over the threshold briefly (thereby triggering the message), but your reputation later dropped back below the threshold. This can happen if a user retracts their vote, if one of your posts got downvoted, or if a post or user was removed.
Note that once you receive a privilege notification message, you will not receive a second message for the same privilege, regardless of how many times your rep goes over the threshold again.