Headings are rendered with incorrect level in a content's "View" tab

Steps to reproduce:

1. Create a new article;
2. On an XML block field - e.g. "Body", type a text, select it and apply the built-in style Heading 1;
3. Publish;
4. Click the article's "View" tab, and using standard browser development tools, inspect the HTML code used to render the text.

Expected behavior: The tag <h1> should be used to render the text since I have selected Heading 1 in step #2
Actual behavior: The tag <h2> is used to render the text.

Please refer to the attached "ez_support_issue_6023_screenshot_1.png" screenshot.