100% height section below navigation

This is a super beginner question, but in the following example why isn’t the “Button Container” section (The blue container under the navigation) truly 100% height? I want it to fill the space below the navigation but it seems to be also adding the height of the navigation making the page scroll.

I have seen this accomplished elsewhere in the forums and I could fix this by just putting the navigation section inside the “Button Container” but I’m wondering philosophically why this set up won’t work. Thanks!