Is the screenshot taken from within the page editor or the live page?
I can see that on your live site the headings underline is in the correct place i.e. centered heading > underline is also centered. In the page editor however the underline may not appear centered under the centered heading text but should do so when you view the live page.
In any case if you want to remove it you can do so in Appearance > Customize > Theme Options and check the box next to ‘Disable Headings Underline’.