There is no set time for updates. There's a check each hour to see if your profile is over 24 hours past an update. If so, it's put in a queue for updates for that hour. Then it takes a little while to process that queue, and that time will vary depending on how much our servers (and Groundspeak's servers) have to do at the moment). That means that your profile gets a new timestamp which is somewhat later than it was yesterday. After 24 hours, you are put in the next update batch, but since that batch takes some time to process, your timestamp will slowly creep forward.