It would probably be easier to interpret if their law defined exactly the number of days you had to reside in HK per year. But Hong Kong is a business city, there are some people that travel a lot for business purposes and their families are still residing in the city.
The extra flexibility can be both a pro and a con though. It leaves it up to bureaucrats to consider what really is ordinarily resident, and there are more ways to challenge in court, and lawyers love that.