Thanks for providing the screenshot, As shown in the Screenshot, the first alert was checked at Oct 26, 2017, 12:19 PM and the Second alert was checked at Oct 26, 2017, 11:15 AM. At that it will increase the threshold value or not. If it is increased then it will triggers an Email alert otherwise not.
An example of drift on a server configured to check Daily:
* at 2PM on Daily the alert condition is checked and found to be true
* emails are queued as tasks, then sent out of the task queue when a background task channel picks them up (there might be other things in the queue ahead of them, e.g. a long running extract refresh that was queued at 1:50 pm)
* alert time is time are logged as when the email goes out, e.g. 2:10
* the following Daily at 2PM the server will look at this alert, and since a Daily hasn't yet transpired (it will at 2:10), it won't check that condition.
Thanks & Regards,