The reason is

The reasons for people keep asking this are:
1. in the old instructions, the countries that you
don't need request police certificate until notified
include HK, US and Canada. Now the new version does
not have US any more. But I still have the old version
including US.
2. A lot of people didn't submit the police certifcate
in their first application and were asked to do so
in the second letter.
That's why it made people confused.