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 behaviour is by design.
Your reputation was indeed over the threshold briefly (thereby triggering the message), but a voter changed their mind, reducing your reputation back below the threshold.
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.