I think they have to update the difficulty algorithm again.
Sometimes miner mine more than 20 blocks in an hour, for example:
504522 - 2017-11-17 10:01    
504547 - 2017-11-17 10:55

Sometimes hardly one block in an hour:
504584 - 2017-11-17 15:51
504585 - 2017-11-17 16:34    
504586 - 2017-11-17 17:34

The Cash Dev team is definitely professional!