China is a big country. It has different population density, terrain and weather conditions in different regions. Looking at China as a whole is not too meaningful. The spread rate is different in different provinces. It is highest in Hubei, the source of the virus and relatively low in the extreme northern and extreme southern provinces. You may be interested to refer to the chart in the following blog.
https://peregrinator2015.blogspot.com/2020/03/blog-post_11.html
The brown line is the spread rate in Wuhan while the red line is the spread rate in the rest of China. The green line is the spread rate for the rest of Asia while the blue line is the spread rate for the rest of the world.
Your question “I'm not sure why Hong Kong have bucked the trend or whether there is something odd about their reporting” has an obvious answer. You are assuming the COVID-19 will spread. Hong Kong is the exception. The virus fails to spread ! Total cases = import cases + spread cases. The majority of the cases in Hong Kong are import cases and there are very few spread cases ! Hong Kong citizens were well trained during the SARS epidemic way back in 2003. Everybody in the streets wear face masks ! They have been snapping up all the face masks in the world ! If you visit the blog again and look at the last photo, these are masks from : (clockwise) Thailand, India, Indonesia, Japan, Korea, cloth, United Kingdom. As a result, spreading only takes place within the family and stops there !