Need to understand why some elements push others away and some don't

Dear WPMU dev Team,

on the screenshot "space 3" you see that the space after me menu on the left is not intended.
On screenshot "space 4" you see the view in the browser. The element on the right is pushing down the Text area thereby creating an unwonted space.
Why is that happening. The Elements are narrow enough to not touch each other. I can add CSS in the Plugin that is producing the shortcode (Formidable Pro). Maybe you can reference me to some source where I can read about why this is happening.

Thank you!
Vassi