Custom layout CSS not included unless it's added to the subtheme .info file?

I created a new layout "mydefault" by copying and adjusting the "hero" layout from the Ohm theme but mydefault.layout.css styling wasn't being included in the site.
After comparing everything with the ohm theme I noticed the generated hero.layout.css was manually included in Ohm's .info file like this:

; Layout Settings
layout[hero][stylesheets][all][] = css/layouts/hero/ohm-hero.layout.css

So after adding:

; Layout Settings
layout[myomegasubtheme][stylesheets][all][] = css/layouts/mydefault/mydefault.layout.css

it was included and worked BUT ... in the ohm example the name between the first brackets is the name of the layout which doesn't work when I do this. It did work however when I set my theme name in the first brackets.

2 questions:

• Is it normal I still have to add layout settings in the info file since I already selected "mydefaul" layout on the theme layout settings-page.
• How come Ohm uses the layout name in the first brackets and I had to use my theme name? (or what am i doing wrong)