So, as you should have found above, it is picking the FIRST section alphabetically.
It was a coincidence it was the one you created first.
Add this to your config.toml
[params]
mainSections = ["sub1", "sub2", "sub3"]
And it should show any of the subsections named.
I went through the quickstart.
By default it is expecting content in a “post” folder, you can see that from the exampleSite provided with the theme.
Check out the homepage index.html layout for exactly how it is working in this theme.
{{/* Define a section to pull recent posts from. For Hugo 0.20 this will default to the section with the most number of pages. */}}