The "per hour" means "in the past 60 minutes". When a defer or failure happens, if there are additional failures in the past 60 minutes, +1 is added to this count.
Lets say you are trying to send out 60 emails, one each minute.
If the first 5 emails go through without an error, then the next one fails, that would be the first failure. So, in the first 6 minutes, you have 1 failure.
The next 10 emails go through fine,
but then 4 emails after that fail. At this point, you have reached the 5 failure limit.
At this point, it has been 20 minutes all together. However, the first failure was 6 minutes in.
The domain sending out the emails would not be able to send any further email until an hour has past since the first failure. In this cause, you would need to wait 46 minutes to send another email, as the first failure would drop off at that point, leaving you with 4 failures in the past hour. I hope this makes sense.