That is because in the CSS, h2 is set up for post and page titles. If you open the post and switch to the HTML tab, what you will find where the h2 fonts are in your post is this at the beginning:
<h2 style="text-align: center;">
What you want to do is change it so that it is like this:
<h2 style="text-align: center; display: block;">
You will have to do that whenever you use the h2 or h1 fonts in posts or pages.
Now, a note on h font tags and SEO.
In general, search engines assign levels of importance by the h font tags. In other words, h1 is most important, h2 next, h3 next, etc. In general, h1 on a home page should be used for the blog title and then h2 for the titles of the content on that page (posts). On an about page, the blog title should switch to h2 and the page title should switch to h1. On the about page the page title is more important than the blog title. (There is debate about all of this.)
So, using an h1 or h2 tag in the body copy on a page or post can create conflicts. Is the title of the page most important, or is "Book me to sing your wedding ceremony!" in the body of the post the most important? Any time you confuse search engines, it is a bad thing.
Sadly many older themes do not take this into account and create h3 and h4 fonts for inside of the body content that is large enough.
Now that I've totally confused you....