Here's the deal.
If you look at "\App_Templates\Default\BlogTemplates\BlogPost.ascx" you will see that content is wrapped with <p> tags:
<p><# Content #></p>
The html code produced by the editor for "format code" contains <div> tags
Now, since <p> elements cannot contain <div> tags (see here
), the <p> tags will automatically be closed,
hence your top text will be inside <p> and the bottom one will not.
I will need to speak with our designers to see if this can be fixed by using <div> tag around content (<div><# Content #></div>) or special handling in code is needed.