Your page is not truly fullwidth because you are using Layout - Fullwidth page template. This layout is limited fullwidth with a container on it. You are supposed to use Blank - No Container| Header, Footer page template.
Please edit your page and in the Page editing screen, change the page template to Blank - No Container | Header, Footer.
You can find it in the Page Attribute meta box:
And if you are editing in Cornerstone, find the Gear icon so that you can get to the Page Settings.
To know more about the different page templates in the theme, please check this out:
Please let us know if this works out for you.