1. It doesn't only there. You can't look at one side only. HK gazettes a street across all its connections, not only one single continuous line. 
2. A three-way interchange can't really be concluded as having a main route by such layout alone. There are many factors. Eg is Shun Long Rd's main route with N Lantay Hwy, despite it being intended to connect with Tung Chung mainly? The logic can't be applied when superimposed (eg to reduce grade-separation span), as in Penny's Bay Hwy side of its interchange with N Lantau Hwy.  