Hi there,
“Interested to know why it isn’t local or combined into existing files”
This is the css layout for the Greennature theme. Gantry compiled the css code based on page layout automatically, example home layout with id 23 that’s why you can see the css prefix layout like theme_name_ID.css where ID is the layout ID.

NOTE: if you want to group all css files into single css file, try to use JCH plugin.