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!