In standard markdown adding two spaces at the end of a line (and a line break of course) leads to a line break </br> in the html.
Doing two line breaks leads to a new paragraph.
But in kirby there are automatic line breaks, so a line break is added even if there are not two spaces at the end of the line. This behavior can be disabled in the config.php with the option markdown.breaks.
So simply add c::set('markdown.breaks',true); in your config.php if you want to have the “original” markdown behavior.