Why are container margins overridden by auto margins?

Hi,

I'm trying to set the left and right margins for a container to 20px but both margins are ending up as "auto" so the margin are not working.

This is the page:
http://vi.anyware.co.nz/residential-flooring-projects/

I can see this code in the generated style sheet (\wp-content\uploads\generateblocks\style-52.css):
.gb-container-c9ab9905{margin-right:20px;margin-left:20px;max-width:1200px;margin-left:auto;margin-right:auto;border-top-style: solid;border-top-width:0;}

The problem child is the "Residential Polished Concrete Floors" section.
This is the spacing for the container:
https://snipboard.io/4Vj1Ja.jpg

Why is there an extra "margin-left:auto;margin-right:auto;" in there and how do I get rid of it please?

Thanks,
Phil